Abstract The interaction between the frequency of vibration and the relative phase between vibration at the seat and the feet on the discomfort of seated subjects exposed to vertical vibration has been investigated in an experimental study. Twelve seated subjects were exposed to sinusoidal vibration at five frequencies (2·5, 3·15, 4, 5 and 6·3 Hz) by means of two vibrators, one under the seat and the other under the footrest. A total of 100 combinations of vibration stimuli with two phases (0 and 180°) between the seat and the footrest at five acceleration levels (0·25, 0·4, 0·63, 1·0 and 1·6 m/s2r.m.s.) and the five frequencies were presented to subjects in two postures (with and without thigh contact with the seat). The subjects judged that the differential vibrations with greater phase difference caused greater discomfort at frequencies up to 4 Hz. The subjects were most sensitive to phase changes at the lowest frequency and the lowest magnitude of vibration. In the equation, ψ=kφn, between the discomfort, ψ, and the magnitude of vibration, φ, the exponent n had a maximum of 1·34 “with thigh contact” and 1·24 “without thigh contact” for the in-phase vibration at around 4 Hz. The exponent had a minimum of 0·63 “without thigh contact” for the out-of-phase motion at 2·5 Hz. The results indicate that vibration discomfort is influenced by the phase between the seat and the feet, but that the effect depends on the frequency and magnitude of vibration and the posture of the body. The phase effect seems to be particularly important with low magnitudes of vibration at low frequencies.