Time-resolved FT-IR studies of two liquid-crystals (4-decyloxy-4’-{(s)-2-methyl hexyloxycarbonyl}tolane and 4-decyloxy-3’-fluoro-4’-{(s)-2-methyloctyloxycarbonyl}tolane) in S-c phase were undertaken. The effects of temperature and voltage on the rate of electric field-induced reorientation of the molecules were studied. According to the theoretical predictions, the electric field does not affect the response time, whereas the influence of the temperature is more complex and cannot be described by the existing theoretical approach. The absolute value of the observed intensity changes (and their sign) during the switching can be explained by the static properties of the samples. A marked intensity reduction of the C=O stretching band in the time-resolved spectra (as compared with the static ones) may be due to the existence of two different conformations of the group in relation to the core. A substituent in the ring (ortho position) directly attached to the carbonyl group splits the C=O stretching band into a doublet in the infrared spectra.