The effect of silicon substrate pretreatment by hydrofluoric acid (HF) upon nucleation density (N-D), surface roughness, and crystal quality of polycrystalline diamond films obtained by chemical vapor deposition has been investigated. Si(100) substrates, initially pretreated classically by ultrasonic scratching with a diamond grit suspension in a methanol bath, have been submitted to HF (50%) etching and subsequent water rinsing prior to plasma exposure. Provided the rinsing time was short, the value of N-D was found to be up to 30% higher and the surface :roughness as much as 30% lower than on unetched substrates, while the diamond film quality remained unaffected. Longer rinsing times in water after HF etching led to a lower N-D than on unetched substrates. The increase in nucleation density on the I-IF-etched substrates has been correlated with an increased plasma etching during the first moments of the deposition process. The increased roughening of the surface due to plasma etching is believed to be responsible for the observed increased nucleation density.