The acidogenic fermentation of the organic fraction of municipal solid wastes; (OFMSW) was studied at thermophilic temperature conditions (55+/-2 degrees C) in a 80 l plug-flow reactor In particular volatile fatty acids (VFA) production and individual acid distribution was monitored at different hydraulic retention times (HRT) (from 2 to 6 days HRT). The feed mixture was composed of OFMSW mechanically selected in a full-scale plant and OFMSW collected from a fruit and vegetables market, with TS content around 25% and a ratio of total volatile solids (TVS)/total solids (TS) of 60%. When inoculum was added to the feed mixture, the thermophilic acidogenic fermentation of the OFMSW became a feasible process for obtaining VFA. The maximum VFA concentration obtained was 19.6 g/l at 6 days retention time, although VFA production at short hydraulic retention time (8-16 h) was already important. VFA production was distributed in all individual acids, but mainly acetic and butyric acids.