The spark decomposition of and of mixtures was studied principally at a gas pressure of 200 kPa. The sparks were generated between a point and a plane either under 50 Hz ac voltage (0.09 J per spark) or by discharging a capacitor (3.59 J per spark). Our attention was only focused on the gaseous by-products: and which were assayed by gas chromatography. The last three compounds were principally observed under the higher energy sparks. Their yields were studied varying the cell preparation technique, the metal constituting the plane electrode (aluminium, copper, stainless steel) and the concentrations of two additives, (between 0 and 1%) and (between 0 and 0.2%). The cell preparation procedure had a strong effect on the formation of all products except ; the yield was for example multiplied by when the cell was carefully dried and outgassed and with an aluminium electrode. The aluminium led, whatever the procedure used, to the highest levels of products. Under the high-energy sparks an increase of the oxygen content of or of the mixture led to a decrease of the and formation rates and to an increase of that of the other compounds. An increase of the content had very little effect on production and led to increased production of and and to a lowering of the formation of other compounds. Under the low-energy sparks the addition of to or to the mixture led to a lowering of the and yields like under high-energy sparks and to an increase of (which became observable) and of . Addition of water resulted in an increase of the and yields, in a lowering of and had no effect on which remained unobservable. Finally it should be noted that the addition of 50% of to the had very little effect on the rates of formation of the gaseous by-products except under low-energy sparks where the mixture led to lower production rates for and .