The objective of the study was the optimization of the retention time in a thermophilic acid digestion system. To achieve this optimization, acidification index defined as the ratio between acidification rate and hydrolysis-rate was used. First-order reaction model developed by Eastman and Ferguson (1981) was used to describe hydrolysis of particulate matter in sludge. The first-order hydrolysis rate constant was found to be 2.36 d(-1), and the initial concentration of degradable particulate COD was found to be 10.4 g/L. A model of acid digestion was proposed that included acidogenic biomass growth and decay. The total acidogenic population was estimated and kinetic parameters of acidogenic population were specific growth rate 6.78 d(-1), net specific growth rate 0.25 d(-1), decay coefficient 0.52 d(-1), and observed yield 0.05 mg biomass/mg COD consumed.,Based on the acidification index, the optimum retention time in a thermophilic acid digester was 3-3.5 d.