... Moscow State University, 119899 Moscow, Russia Samples of PbS x Se y Te 1-x-y , Pb 1-x Sn x Te 1-y Se y and Pb 1-x Sn x Te 1-y S y quaternary solid solutions were investigated in the 4-200 K temperature range using electrical and X-ray methods. The regions where low-temperature phase transitions take place were established. It is shown that phase transitions in these solid solutions are associated with off-center S and Sn ions. ... Phys. Solid State, 1983, v. 25, p. 2055. ...