Infrared (IR) absorption spectra of semiconducting Ba1-xKxBiO3 crystals (x = 0, 0.07, 0.17) were recorded in the vicinity of the band edge at temperatures ranging from 6 to 490 K. The measurements reveal a sharp anomaly in the behavior of the material near T-a = 65-75 K. At this temperature the band gap in all investigated specimens narrows, inducing a step-like decrease of IR transmittance. The difference in transmittance above and below T-a depends on x and for x = 0.17 exceeds one order of magnitude. Recorded changes are associated with the low-temperature structural phase transition in the compound. They resemble a switching phenomenon controlled by the transformation of crystallographic symmetry in the system with substantial electronic instability.