A wet chemical procedure has been elaborated to measure the thickness of thin silicon dioxide layers. The procedure is based on the etching of the SiO2 layer by HF and the determination of Si concentration in the microgam per liter range in the HF containing etch solutions. Two analytical techniques were optimized for this purpose: a spectrophotometric technique, the so-called molybdenum blue method and inductively coupled plasma mass spectrometry (ICP-MS). In the first method a detection limit of 3.3 mu g/L Si could be achieved with a sensitivity of (780 +/- 8.7) x 10(-6)/(mu g/L Si). Interference by HF up to 0.1% v/v (volume/volume %) HF could be eliminated by adding boric acid to the solution. In the second method Si was determined by ICP-MS using the Si-28 isotope. The detection limit in bidistilled water was 1.2 mu g/L Si with a sensitivity of (5807 +/- 98) cps/(mu g/L Si). The presence of HF increased the background signal of Si due to the etching of the quartz plasma torch. In 0.005% v/v HF a detection limit of 5.9 mu g/L Si could be achieved. For silicon dioxide layers below 1 nm, a reproducibility better than 5% was obtained.