This experiment was carried out to evaluate the effects of formic acid and formic acid plus molasses additives on the grass silage fermentation characteristics and degradability of dry matter (DM) and acid-detergent fibre (ADF). Grasses were cut in early (experiment I), middle (experiment II) and late maturity (experiment III). The additive treatments in experiments I, II and III were: 1) Untreated; 2) 0.5% Formic acid (FA); 3) FA+2% molasses; 4) FA+4% molasses; and 5) FA+6% molasses. The silages were ensiled in jar silos (1 L) and DM and ADF degradability were determined by using the nylon bag technique in the rumens of four fistulated sheep. Increasing formic acid plus molasses decreased the ADF and neutral detergent fibre (NDF) content of silages as compared with FA and untreated silages in experiments I, II and III (p < 0.001). Formic acid plus molasses silages were well preserved with low pH (4.62, 4.51, 4.54), high lactic (30.62, 32.20, 28.54 gkg-1 DM) and acetic acid (19.21, 21.46, 20.65 gkg-1 DM) and low butyric acid (3.81, 2.19, 1.43 gkg-1 DM) as compared with untreated silages (pH: 5.12, 5.35, 5.36; lactic acid: 20.70, 21.47, 15.21gkg-1 DM; acetic acid: 14.32, 15.88, 11.28 gkg-1 DM; butyric acid: 4.50, 3.47, 6.68 gkg-1 DM in experiments I, II and III, respectively). However, silage fermentation quality was not at the optimal level. The FA silages were moderately preserved with more restricted fermentation than the formic acid plus molasses and untreated groups. In experiments I, II and III, significant differences were noticed between untreated and other silages in respect of DM degradability. ADF degradability was not affected by the silage treatments at 4, 16 and 24 h, but was significantly (p < 0.01 or p < 0.001 ) affected at other times.