The effect of phosphorus and titanium additions on the accumulation of vacancy defects in Cr16Ni15Mo3 austenitic stainless steels under electron irradiation at room temperature is studied by positron annihilation spectroscopy. It is shown that, at this temperature, phosphorus has no noticeable effect on the accumulation of vacancy defects. This is due to the low mobility of vacancies and the low concentration of impurities. Titanium, due to its high concentration, enhances the accumulation of vacancy defects during irradiation, but this effect is weak.