This study aims to investigate the storage response of an activated sludge system in the presence of two different substrates which are stored as two different polymers. The objective of the study was to determine the changes in the response of an activated sludge system when two different storage mechanisms could occur simultaneously. Acetic acid (HAc) and soluble starch (SolS) were selected as model substrates and three different feeding conditions, namely (i) when both HAc and SolS were initially present in the reactor, (ii) only HAc was present, and (iii) only SolS was present in the substrate solution. The batch reactors were monitored for acetic acid, polyhydroxyalkanoates (PHA), poly-glucose (glycogen like substances) and oxygen uptake rate (OUR). The experiments have shown that, while the main portion of hydrolyzed starch was stored as poly-glucose, which was further used for heterotrophic growth, the rest was utilized for direct growth. However, acetic acid was totally stored as PHA and the stored PHA was used for biomass growth under the presented experimental conditions. When the system was fed with the substrate mixture, the storage mechanism was not significantly affected. Both PHA and polyglucose storage took place simultaneously with the same stoichiometry and kinetics defined for single substrate utilization.