Formulation is a term used in various senses in various applications, and its fundamental meaning is the putting together of components in an appropriate relationships or structures, according to a formula. Several prepared formulations for chlorpyrifos-ethyl and lambdacyhalothrin as Emulsifiable Concentrate (EC) formulation were subjected to WHO chemical tests to find out which formulation is matching the requirements. The physicochemical properties of the standard commercial formulation versus the prepared formulation were determined. The emulsion stability test was determined according to WHO specification method. Besides, the determination of free acidity and alkalinity as well as, viscosity and surface tension. The results of emulsion stability test indicated that the ml separation in the prepared formulation of chlorpyrifos was 0.1 ml cream after 1 hr in the case of hard water and 0.9 ml in soft water for chlorpyrifos 48% + Calcium dodecyl benzene sulfonate  (CaDBS) 2.6% + Triton X-100 2.4% (F1) compared to < 0.1 ml and < 0.3 ml in hard and soft water respectively in the second formulation which consists of chlorpyrifos 48% + dodecyl benzene sulfonate (CaDBS) 2.8% + Triton X-100 2.2% (F2).The most successful prepared formulation was (F3) which consists of chlorpyrifos 48% + Calcium dodecyl benzene sulfonate (CaDBS) 3% + Triton X-100 2% with < 0.05 ml in both hard and soft water. While the ml separation in the commercial formulation of chlorpyrifos was 0.1 ml in hard water and 0.2 ml in soft water. The cream separation in emulsion stability test of prepared formulation of lambda-cyhalothrin was not observed after 1 hr in both hard and soft water for lambda-cyhalothrin 5% + dodecyl benzene sulfonate (CaDBS) 1.8% + Triton X-100 3.2% (F1) so it was the most successful prepared formulation compared to 0.9 ml in hard water and 0 ml in soft water in the second formulation which consists of lambda-cyhalothrin 5% + dodecyl benzene sulfonate (CaDBS) 2% + Triton X-100 3% (F2). While the ml separation in the commercial formulation of lambda-cyhalothrin was 0.4 ml cream + 0.3 ml oil in hard water and 0.7 ml cream + 0.2 ml oil in soft water. The most successful formulations were tested against susceptible and field strains of Spodoptera littoralis (Boisd.). The efficiency of the two prepared formulations were determined; in comparison with the standard
(commercial) formulations of chlorpyrifos-ethyl and lambda-cyhalothrin respectively, against the susceptible and field strains of S. littoralis (Boisd.). Also, the effect of adding the synergist (Sylgard 309®) to each treatment was determined. In the susceptible strain after 48 hours bioassay; LC50 value for the prepared formulation of chlorpyrifos was10.12 ppm and after adding Sylgard 309® the result was found to be 7.14 ppm, while LC50 values for its commercial formulation alone and/or with Sylgard 309® were 13.74 and 9.61 ppm, respectively. LC50value for the prepared formulation of lambda-cyhalothrin was 85.70 ppm; after adding Sylgard 309® became 68.34 ppm, while LC50 for the commercial formulation of lambdacyhalothrin alone and/or with Sylgard 309® were 152.54 and 108.52 ppm, respectively. In the field strain after 48 hours bioassay; LC50 value for the prepared formulation of chlorpyrifos was 17.89 ppm, but with the addition of Sylgard 309®the result showed a magnificent effect as of 13.77 ppm. For its commercial formulation only and/or in addition to Sylgard 309® LC50 values were found to be 163.19 and 154.28 ppm respectively. LC50 value for the prepared formulation of lambda-cyhalothrin was 1212.62 ppm; with the addition of Sylgard 309® was slightly decreased to 1208.23 ppm, while LC50values for the commercial formulation only and/or in addition to Sylgard 309® were 1925.87 and 1914.77 ppm, respectively. The calculation of Resistance Factor (RF) after 48 hours bioassay revealed that the field strain compared to the susceptible one of S. littoralis showed a real case of resistance to both of the commercial formulations of chlorpyrifos and lambda-cyhalothrin while the prepared formulations of each showed either the natural tolerance or the vigor tolerance case, in respect.