The interfacial thickness between immiscible polymers, bisphenol A polycarbonate (PC) and styrene-acrylonitrile random copolymers (SAN), was measured by ellipsometry as a function of temperature and acrylonitrile content in SAN, in order to study the miscibility between PC and SAN. Both the temperature and copolymer composition (AN content) dependence of the interfacial thickness in the equilibrium state indicated a maximum. Using the values of the interfacial thickness, it is possible to calculate the polymer-polymer interaction parameter chi(AB) between PC and SAN. The chi(AB) exhibited the lowest positive value at about 200 degrees C for each copolymer composition due to the maximum of the interfacial thickness, i.e. at low temperature range, chi(AB) decreased with increasing temperature, then chi(AB) increased at high temperature range. This behaviour implies the possibility of the coexistent behaviour of upper and lower critical solution temperature (UCST and LCST) in the phase diagram of the mixture consisting of polymers with suitable molecular weight. This coexistent behaviour of UCST and LCST was confirmed by measuring cloud point curves of the mixtures having lower molecular weight PC. On the other hand, the copolymer composition dependence of chi(AB) also has a minimum in a certain copolymer composition, which leads to the miscibility window behaviour. From these results, it was shown that there appeared to be a loop-type miscibility window ('miscibility egg') in the miscibility diagram of copolymer composition versus temperature of PC/SAN. From the calculation based on these results, it has also been shown that all three segmental interaction parameters chi(i/j) of PC/SAN, i.e. chi(S/C). chi(AN/C) and chi(S/AN), decreased with increasing temperature monotonically. It was predicted that the LCST behaviour in PC/SAN mixtures could occur due to the remarkable decrease of interaction chi(S/AN) between comonomers in SAN at high temperature, without the free volume contribution.