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Soil nitrogen balances involving lysimeter experiments and cropped and uncropped pot experiments have shown that in many cases such important nitrogen deficits were observed that there must be hitherto unknown or unsuspected pathways along which nitrogen escapes. In former experiments of the senior author it was shown that in acid soils, dressed with ammonium sulphate, nitrogen can be lost as N—O compounds during nitrification. In the present investigation more accurate determinations made it clear that when the pH of a liquid culture of nitrifying bacteria dropped below about 5.5 nitrogen was not only lost as N—O compounds, but to a greater extent as nitrogen gas, most probably by chemical reaction between the HNO2 formed during nitrification and the ammonia present in the solution. In pot experiments with acid sandy soils from different parts of the Netherlands, losses of up to 74% of the ammonium sulphate added were observed. An investigation was made of the conditions which promote or are essential for these losses. When the initial pH of the culture medium, be it a solution or soil, enables the nitrifying bacteria to develop and the buffer capacity is of such a magnitude that the pH drops below 5.5 during nitrification, volatilization is to be expected. As volatilization and ammonification often coincided, both processes seem to be linked in some way or another; volatilization, however, has also been observed in the absence of ammonification. These losses are not caused by evaporation of ammonia, in which case they should increase as the pH increases; the contrary happens, however, and the losses stop entirely when sufficient CaCO3 is added to keep the pH above 5.5. They are also not due to denitrification, as they do not occur when nitrogen is added as nitrate instead of ammonium sulphate. Also there are no losses when the soil is pasteurized, which proves that they originate in bacteriological processes.