A 'semi-solid' Fenton process (SSFP), in which the Fenton process (FP) takes place in a matrix with a low liquid-to-solid ratio (</= 5:1), was conducted on the degradation of o-nitroaniline (ONA) in hazardous solid waste. Based on the assumption of a first order reaction between ONA and the hydroxyl radical (.OH), an SSFP kinetic model was developed to accurately describe the kinetics of ONA degradation and to quantitatively investigate the effect of operating conditions. Half-life prediction model was also proposed and compared with the half-life calculated from pseudo first order equation to prove the effect of operating conditions. It showed that the degradation of ONA proceeded rapidly at pH 3.5. The kinetic rate constant, k, for ONA degradation increased with the initial ferrous ion concentration, but decreased with the liquid-to-solid ratio. With respect to the lowest residual ONA concentration, the optimal ferrous ion concentration and liquid-to-solid ratio were 0.062 mol/kg dry weight and 1.8, respectively.