Positron annihilation lifetime studies (PAL) have been carried out on Al-Cu alloy, in order to get information about solute surroundings at the annihilation site. Exceedingly large numbers of positrons are found to annihilate at the matrix-Al2Cu precipitate interfaces. The defect structure of AlCu alloy with a variety of Cu concentrations is deduced from the PAL measurements at room temperature. The change of the average positron lifetime with different Cu concentrations (up to 25 wt. % Cu) has been discussed. PAL measurements revealed saturation trapping of positrons at large concentration of copper in the alloy, specifically in matrix-precipitate interface. Complementary techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), and Vickers hardness are used in the present work. The nucleation and growth of the Al2Cu particles lead to reduction in the positron lifetime and raising in the Vickers hardness. Hence, the hardness variation is qualitatively consistent with that of positron annihilation parameters. The information obtained from PAL studies provides direct observations of positron trapping at the misfit interfaces between the Al-rich matrix and the Al2Cu phase.