RXTE/PCA observations of several black hole X‐ray binaries in the low spectral state revealed a tight correlation between spectral parameters and characteristic frequencies of variability of X‐ray flux. In particular, the amplitude of reflection increases and the slope of Comptonized radiation steepens as the noise frequencies increase. The data also suggest that there is a correlated increase of the width of the Fe fluorescent line, probably related to the Doppler broadening of the reflection features. Moreover, the width of the line seems to follow&Dgr;E/E ∝ vQPO1/3law. If confirmed with higher energy resolution observations, this result will have significant impact on the models of the accretion flow and on our understanding of the nature of characteristic frequencies of variability in X‐ray binaries. In particular, it will lend support to the truncated disk picture, confirming that the spectral variations are indeed related to changes of the position of the inner disk boundary and that characteristic variability frequencies are proportional to the Keplerian frequency at the inner boundary of the accretion disk. © 2004 American Institute of Physics