The dependence of electron density and ion flux on radiofrequency (RF) power has been measured in a 2 + 27 MHz dual-frequency capacitive discharge with silicon electrodes at 6.7 Pa gas pressure. In Ar/O₂ mixtures the electron density and the ion flux vary in a very similar way (i.e. their ratio, υ, is constant), in good agreement with the simple electropositive transport theory. Both 27 and 2 MHz RF powers have a significant effect on the plasma density and the ion flux. The effect of the 2 MHz power is likely a combination of enhanced plasma heating by dual-frequency excitation and ionization caused by secondary electron beams, which are known to be produced efficiently at oxidized silicon surfaces. In contrast, in Ar/C₄F₈/O₂ mixtures such as those used for industrial dielectric etching, υ is always bigger than the theoretical electropositive value, and becomes very high when the ratio of 2 to 27 MHz power is high. Under these conditions the electron density is very small, whereas the ion flux remains considerable. We attribute the increased plasma transport to the presence of a significant density of F⁻ negative ions, combined with increased penetration of the 2 MHz electric field into the plasma bulk at high 2/27 MHz power ratios.