An electron theory of photoinduced phase transitions and cavityless optical bistability in a light field with a finite optical-spectrum width is constructed. The Liouville equation for the density matrix is used to derive the existence criteria and calculate the main characteristics of the given phenomena. Broadening of the optical spectrum is shown to reduce the possibility of observing critical features (the existence criterion becomes more stringent and the hysteresis-loop area becomes smaller). Finally, results are compared with experimental data for CdS and amorphous GeS 2 .