The Ar dilution effects on diamond deposition at moderate pressures (26.6-400 Pa) in an electron cyclotron resonance hydrogen-methane plasma have been studied in connection with electron temperature (T-e) and electron density (n(c)). The double probe measurement revealed the dependence of T-c and n(e) on Ar concentration, pressure, and microwave power. T-e decreased in proportion to Ar concentration and was in the range of 3.7-7.5 eV. In contrast, n(e) exhibited only a small change with increasing Ar concentration except at a higher pressure and was on the order of 10(10)-10(11) cm(-3). The Ar dilution promoted nucleation rather than growth. as shown by an increase in nucleation density and renucleation on preexisting diamond grains. The mechanism leading to the peaked growth rate at 33-50 vol.% Ar was explained by the variation of the ion-bombardment energy and the fluxes of radicals, based on the measured T-e and n(c). (C) 2003 Elsevier Science B.V. All rights reserved.