Abstract The imprecise nature of reaction and photolysis rates used in photochemical models can introduce significant uncertainties on computed concentrations. In this paper, we use a box photochemical model of hydrocarbons in the atmosphere of Neptune. The scheme of reactions derived from Dobrijevic (1996) contains 32 species and 189 reactions. The set of continuity equations is integrated with the method of Gear (Gear, 1971). A Monte-Carlo technique is used to introduce random uncertainty in rate coefficients and to estimate uncertainties in computed concentrations. Significant deviations from mean value have been obtained: 100% for C 2-compounds, 200% for C 3-compounds and more than 2000% for higher hydrocarbons. Implications for one-dimension photochemical models are discussed.