The heat and smoke spread during thermal decompn. and combustion of modified and unmodified polyester materials in an open scheme: room-corridor in 1(st) stage of fire development were analyzed as functions of the fire time. As flame retardants of polyester materials, MoO(3) and Sb(2)O(3) (concn. 14% by mass) were used. For the detn. of fire parameters, the modified com. computer program CFAST based on a zone model was used. The effect of fire retardants on upper and lower layer temps., level of upper smoke layer above floor surface and the range of visibility in both layers was studied. The calcd. values of the fire parameters indicated a decrease of heat accumulation in fire environment during combustion of polyester with the fire retardants in relation to unmodified polyester material. The retardants (as additives) reduced the hazards of declining the hot smoke layer in relation to pure polyester but brought an increasing danger connected with the visibility reduction.