The effect of pH of a nitrite solution on the depletion rate of nitrous acid was experimentally investigated. The depletion reaction of nitrous acid proceeded when the initial pH of a nitrite solution was less than 4.6. When the nitrous acid concentration was greater than 10(-3) mol . l(-1), the ratio of nitrite consumption to nitrate production was approximately 3. However, when the nitrous acid concentration was smaller than 10(-3) mol . l(-1), the ratio of the nitrate production to nitrite depletion became greater than 3 due to desorption of N(2)O(4).