Biodegradation of solution-cast starch films by Bacillus subtilis was monitored using a quartz crystal microbalance (QCM). A starch film was formed on the crystal by solution casting and exposed to the Bacillus subtilis culture in a bioreactor. The high sensitivity of the QCM could monitor small weight changes of the starch films on the crystal in the initial stages of biodegradation by secreted exo-enzymes of the bacterium. The feasibility of this approach as a means of quantification and characterisation of biodegradability of different polymeric materials by selected organisms is discussed.