This study examined the stability of active ingredient (a.i) and three different commercial formulations of metribuzin herbicide (suspension concentrate (SC), water dispersible granules (WG) and wettable powder (WP)) after exposure to UV-rays, direct sunlight, different temperature degrees, and different pH values at different intervals. In addition, the effect of accelerated storage at 54 2 oC for 14 days under laboratory conditions and the degradation rate of metribuzin for test samples were evaluated. The metribuzin content and degradation products were determined using GC–FID and GC–MS. Results indicated that direct sunlight and high temperatures were more effective than ultraviolet rays in accelerating the rate of metribuzin degradation. In contrast, the degradation rate of metribuzin was faster in acidic and alkaline pH than in neutral medium. The formulations of metribuzin are stable after accelerated storage at 54 2 oC. There were no discernible differences in metribuzin content before and after 14 days of storage. Metribuzin formulation in SC was more stable than metribuzin in formulation WG and WP, but overall, formulation type had a limited effect on the metribuzin degradation rate. Analysis of metribuzin a.i after exposure to direct sunlight using GC-MS identified four photodegradation products: deamino metribuzin (DA), diketo metribuzin (DK), 6-(tert butyl) -1,2,4-triazin-5(4H)-one and 4-amino-6-(tert butyl)-3- (methylsulfonyl)-1,2,4-triazin-5(4H)-one.