Autohydrolysis of softwood chips, used as raw material in a paper mill, was studied to simulate one of the aspects of industrial hydrolytic steps of a thermomechanical pulping process. The compositions of the pulps and of the hydrolysates were determined as a function of the residence time and the severity of the treatment. It was found that acidic arabinogalactans were largely affected and depolymerized, even at low severity (log(R-0) < 2). The percentage of residual arabinans in the pulp after hydrolysis seemed be a good indicator of the severity of the treatment and can provide an effective guide for an optimization of the industrial process. A hydrolytic step performed at a severity of log(R-0) = 1.8 resulted in the extraction of similar to 100% of the water-soluble acidic arabinogalactans, which constitutes the main part of the "anionic trash" released in mechanical pulping.