This article presents the results of research work associated with the assessment of the course of composting consisting in development of a model used to define in real time the respiration indicator (AT4). The value of AT4 determines the level of biological stability based on forecast change of amount of the organic substance used in a biological process performed in aerobic conditions. The model was developed assuming that there is correlation between AT4 and OM (organic matter). Test results confirmed high value of correlation between both variables equal to 0.88743 computed based on Pearson’s method. Value of analogous coefficient of correlation between AT4 and OMc (the computed amount of consumed organic substance) was −0.9006. To describe the change of the organic substance amount over time the first-order kinetics equation was used for which the value of the base process speed constant k(d−1) fixed by means of mathematical modelling was 0.00378. Using equation of kinetics for computation of OMc values, and comparing them with real values, mean error of 0.45 ± 0.34 was obtained. As a factor correcting value k during the process, a coefficient dependant on the on-line monitored values of temperature and oxygen content in compost pores was introduced. Field research was performed at a composting plant for mechanically dehydrated sewage sludge with an addition of structural material located in Słupsk (Poland).