Composting pine bark alone and with additives is an interesting alternative to recycling waste as compost. Our study was focused on the exploration of various sulphur fractions (total, plant available, easily mineralisable organic and residual) in four composts during a progressive composting process. Composts used for the study were prepared using Scots pine (Pinus silvestris L.) bark and chopped green plant material (GPM, i.e., a mixture of green parts of buckwheat (Fagopylum esculentum L.), peas (Pisum sativum L.), serradela (Ornithopus perpusillus L.) and vetch (Vicia sativa L.) harvested before flowering). They were prepared according to the scheme: C1 - pine bark, C2 - pine bark mixed with urea (a dose of urea equivalent to 1kg N per 1 m³ of pine bark), C3 - pine bark mixed with GPM (0.5 Mg of GPM per 1 m³ of pine bark) and C4 - pine bark mixed with GPM (3.5 Mg of GPM per 1 m³ of pine bark). The composting process lasted 203 days and comprised 6 stages. It was found that compost prepared from pine bark and green plant material (C4) contained the highest amounts of sulphur and their changes were significant during the composting process. Although it is not routinely applied in such studies, the use of PCA to summarize the influence of composts and stages was found to be a valuable tool. The PCA data proved that with regard to the plant-available sulphur and easily mineralisable organic sulphur, the composting process could be shortened to 80 days with no deterioration of compost quality. Total and residual sulphur contents showed a similar pattern. The amounts of sulphur extracted with CH₃COOH and KCl as well as and their changes observed during the composting process were comparable. However, the solution of KCl may be considered as a more sensitive extractor of sulphur in composts.