An atmospheric-pressure flames has been sampled continuously through a small orifice into a vacuum system housing a mass spectrometer. For this, flat flames were burnt on an insulated metallic burner, so that electric fields could be applied between the burner and the metallic sampling nozzle, at the tip of which was an orifice of diameter <0.23 mm. Detected ion currents vary with these applied voltages; positive ions have maximum current for applied voltages close to zero, but the currents of negative ions are greatest when the sampling nozzle has a small positive voltage (2–25 V) with respect to the burner. The observations are explain in terms of sheaths of either positive ions or electrons blanketing the metal around the sampling orifice. Moreover, these sheaths of ions can under certain conditions cover the inlet hole. It is concluded that optimal sampling conditions for ions require the application of a voltage to maximize each observed ion current. Further experiments were performed to collect either all the positively or negatively charged species, including electrons, which pass through the sampling orifice. These total currents were collected on a needle inside the vacuum system, immediately behind the sampling orifice. Total ion currents were measured as a function of both the voltage difference between burner and nozzle, and also between the nozzle and the collecting needle. It is clear that if the electron density is very low (∼10 7 /ml) only a small proportion of the electrons pass through the orifice, but instead are collected on the sampling nozzee. However, for large electron densities (∼10 10 /ml) a large fraction of them actually pass through the orifice into the expansion duct just inside the first vacuum chamber. This contrasts with positive ions, which pass through the inlet orifice without significant loss. The same applies to negative atomic or molecular ions. The governing feature is the thickeess of the ion sheath and ia discussion is given of the factors determining its magnitude.