The quality of silage is of great importance because it benefits animal production, animal health and food quality. This study examined the impact of mixtures of sodium benzoate, sodium nitrite, hexamine (HMTA), sodium propionate, sodium bisulphite, and propionic acid on silage quality in low- and high-wilted grass/clover mixtures. Additionally, it was focused on the nitrite concentration in the silages. The silage (wilted to 300 or 600 g dry matter kg-1 of fresh weight) consisted of 50% red clover (Trifolium pratense) and 50% timothy (Phleum pratense). The forage was not chopped, and was ensiled in two types of silo: 25-litre stainless steel laboratory silos, stored for 120 days and 1.7-litre glass silos, stored for 14 days. The effect of silage additives in improving the fermentation process, reduction of clostridial growth and silage losses was achieved in low-wilted silages whereas the impact on aerobic stability enhancement was pronounced in both low and high dry matter silage. The combination of sodium benzoate and sodium bisulphite had an unsatisfactory effect on the reduction of nutrient losses and improvement of aerobic stability. A mixture of 672 g sodium benzoate, 425 g sodium propionate, and 1890 g propionic acid effectively improved the silage quality. The application of 465 g sodium benzoate, 360 g sodium nitrite, 240 g hexamine, and 165 g sodium propionate at a dose of 3 litres per tonne fresh matter was sufficient to inhibit clostridial and yeast growth and increase silage stability in low-wilted forages, whereas a mixture of 698 g sodium benzoate, 540 g sodium nitrite, 360 g hexamine, and 248 g sodium propionate at a dose of 4.5 litres per tonne was more effective at higher dry matter. The nitrite-N concentration in silages was far below the toxic level. © 2004 Taylor & Francis.