In this study, dielectric spectroscopy was utilised to evaluate and define the optimum harvesting time for polyhydroxyalkanoates (PHA) production. It is essential to harvest PHA at the optimum time during fermentation for maximum yield, otherwise cells start degrading. Two carbon sources (acetic and butyric acids) were used in laboratory based experiments and a number of samples were measured ex situ for PHA production. The real-time measured capacitance in addition of identifying the cells growth phase, it correlated very well with ex situ measured PHA produced within the cells. The probe has proven to be a useful tool to assess process kinetics, to monitor real-time cell growth, PHA produced and defining the optimum harvesting time.