  We study energy dependencies of quasi-periodic oscillations from a number of
black hole X-ray binaries. The selected sources were observed by RXTE at time
periods close to state transitions and showed QPO in the 1-10 Hz range. We have
constructed QPO root-mean-square energy spectra, which provide information
about underlying physical processes leading to QPO generation. These spectra
show an interesting anti-correlation with the time averaged spectra. The QPO
r.m.s. spectra are harder than the time averaged spectra when the latter are
soft, while they are softer than the time averaged spectra when the latter are
hard. We then discuss these observational results in the context of simple
spectral variability models. Hard QPO spectra can be produced by quasi-periodic
modulations of the heating rate of the Comptonizing plasma, while soft QPO
spectra result from modulations of the cooling rate by soft photons.
