The present paper is concerned with turbulent jet flows in which the density varies due to exchanges of mass. Such flows have gained some attention over the last few years but, to our knowledge, this is the first time results are reported for the influence of density variations on velocity spectra. Our measurements are obtained by laser–Doppler anemometry and part of the paper is also devoted to testing several procedures for estimating spectral densities from the randomly sampled data. Comparisons between turbulent helium and air jets confirm that the lighter gas jet develops at stations nearer from the exit nozzle and that the turbulent Reynolds number Rλ remains the essential parameter to describe the distribution of turbulent scales in strong variable density jets.