Measurements are reported on the infrared (IR) absorption, the optical band gap, and the dark conductivity of amorphous silicon‐phosphorus alloy thin films (a‐Si:P) with 20–44 at. % P prepared by coevaporation of Si and P. The results show that the optical band gap can be tailored in a range of 1.5–2.15 eV by varying the P concentration and the annealing temperature. The band gap for the sample with 20 at % P is the widest (1.70–1.82 eV) when annealed at temperatures ≤600 °C. From the IR‐absorption study, a stretching mode associated with Si—P bonds at 465–474 cm−1has been found for all alloy films. An evolution (shift or sharpening) of the Si—P absorption band with annealing temperature and P concentration have also been observed. Based on the IR‐absorption study the change of bonding structures in the alloy films is discussed. Conductivity measurements show that two electron conduction processes mainly exist in the investigated temperature range: extended‐state conduction in the conduction band at high temperatures and hopping conduction in the band tail at low temperatures. The transition temperature is around room temperature (14–55 °C), indicating that the carriers may mainly drift by hopping even at room temperature. The correlation between the optical band gap and the bonding structure is discussed. ©1995 American Institute of Physics.