Determination of

We present new measurements of the interstellar reddening parameter $R_V=A_V/E(B-V)$ towards 35 O stars. The results are combined with measurements from the literature for 60 stars to study the behaviour of RV with heliocentric distance. RV is the single basic parameter which characterizes the interstellar extinction from the near-infrared to the far-UV spectral region. The absolute extinction AV, from which RV is derived, is best determined by optical and near-infrared photometry (Cardelli et al. [CITE]). We consider important the derivation of RV with the same technique in the direction of as many as possible galactic O stars