Humic acids (HAs) from soil and compost at the beginning (SO) and at the end of the stabilization process after 130 days (5130) have been fractionated by coupling size exclusion chromatography (SEC) and polyacrylamide gel electrophoresis (PAGE). Preparative quantities of HA fractions (HAFs) with different molecular sizes (MSs) and exactly defined electrophoretic mobility (EMs) have been obtained from all samples and the HAFs weight content has been studied. A high degree of similarity in HAFs weight content between soil HA and a stabilized compost HAsS130 has been observed. Such data seem to be reliable for monitoring the evolution of the compost organic matter to humic substances for their agricultural uses. (C) 2001 Elsevier Science Ltd. All rights reserved.