It is well known that the electrical conductivity sigma of polymethylmethacrylate (PMMA) is highly sensitive to the nature of the salts used as dopants. In the present work, the effect of doping this polymer with different salts such as copper acetate, cobalt acetate and ferric oxalate as organic salts and chromium chloride, copper chloride as inorganic salts was investigated by means of positron annihilation lifetime technique (PAL). Furthermore a correlation was established between the free volume, (PAL) parameters I-3,tau (3) and the electrical conductivity a of the doped samples as well as distributions of the free volume holes. It has been found that for PMMA doped with organic salts a increased by 100% in comparison with the corresponding value of plain PMMA. On the other hand, with inorganic salts sigma decreased by 70%. These variations were interpreted in terms of the free volume distributions and the correlation between sigma and tau (3), I-3 and tau (2) parameters.