Five grape marc composts prepared by different procedures (composting and vermicompost at several scales) were tested as potential components of plant growth media. The five composts had high organic matter content (>90%), low electric conductivity (<1 dS m(-1)) and a pH between 7 and 8. Different chemical and biochemical analyses performed indicated the higher stability of those composts submitted to a longer composting process or to a vermicomposting process (lower water soluble organic matter, respiration and dehydrogenase activity). In order to determine the suitability of the composts as substrate components, plant growth tests were performed by blending the composts with peat or commercial substrate at two compost rates (25% and 50%). The mixtures were sown with barley (Hordeum vulgare L.) and grown under controlled conditions in an incubation chamber. No prejudicial effects derived from the use of composted grape marc were observed, whatever the procedure of composting used. The results showed that four out of the five composts would be suitable for use in plant growth substrates elaboration, as they did not reduce productivity with respect to the control substrates.