The lattice-gas model of Schouten, ten Seldam and Trappeniers is exten
ded to mixtures of polymers. This model allows for three options on ea
ch lattice site: a molecule of type one, a molecule of type two or a h
ole. It describes a binary gas-liquid system in the molecular-field ap
proximation. Following Flory, the chain length r is introduced as addi
tional parameter in order to express the fact that the molecules occup
y several lattice sites. In this paper we introduce two Flory chain le
ngth parameters (leading to the model of Tompa) in order to describe p
olymer mixtures in the presence of their vapor phase. The model can al
so be applied to a ternary mixture consisting of two polymers in a com
mon solvent. The conditions under which the critical line becomes self
-crossing are given and explicit expressions are evaluated for the coo
rdinates of this point. The critical line is displayed for a number of
such cases in the pressure-temperature projection.