The optical properties of AmB%?-type layer-and-chain crystals a r e not clearly understood up to now. Published data /1 to 6/ a r e episodical, contradictory and have been discussed irrespective of the low-temperature structural phase transitions typical of this class of compounds.In this report we confine ourselves to setting forth some new results obtained while studying the effects of intercalation, uniaxial pressure, temperature (5 to 180 K), composition variations, and structural phase rearrangements on exciton absorption spectra of single crystals of A1' B1%F -type ternary layer semiconductors (T1GaS2, T1GaSe2, T11nS2) and solid solutions based on them. As a result of thorough measurements it has been revealed that the nature of the thermal shift of the exciton absorption band is very complex:specifically, when the light is directed transversely to the cleavage plane of a TlGaS2 crystal (d = 23 p m ) the temperature coefficient for the ground state (n = 1) of a direct exciton has an unusual positive sign and varies in absolute value from l~l O -~ to 2. 7x16-4 eV/K. A s this takes place, the exciton absorption peak is displaced from 2.606 to 2,637 eV as the temperature increases from 4.2 to 180 K, but the trend of the Eex(T) dependence is quite different from that r eferred to in /4/. Actually, uniaxial compression perpendicular to the layers is essential, but in the opposite direction pointed out in /4/ it affects the energy positions of the exciton ground state and the values of their temperature coefficient, whereas in certain pressure and temperature regions it even leads to a reversal of the coefficient sign (see Tables 1 and 2).A s this takes place, the value and the sign of the differential baric displacement coefficient of the exciton absorption band depend on the temperature and pressure regions (see Table 3).