Gas flow in long, narrow, very shallow, micromachined silicon dioxide–quartz glass channels was studied with the use of a mass spectrometric system. The channel bottom was partly covered with an evaporated, 10-Å-thick platinum film along its whole length, and the channel was inserted as the only leak between a gas mixing chamber and an ultrahigh vacuum mass spectrometer chamber. At a wide pressure range upstream of the channel (10–1000 Torr), the steady state flow through the channel was characterized as being close to molecular. At a pressure buildup of either H2or O2in the gas mixing chamber, the mass spectrometer response was delayed more than what can be explained by molecular diffusion. A total restriction of the diffusion due to adsorption on platinum is suggested. The assumption is in accord with different validation experiments. A model for the adsorption delay time is presented and discussed. ©1997 American Institute of Physics.