This paper presents a new method of studying mould activity as a function of water activity by measuring the heat produced by the fungal metabolism. During a measurement a small sample (<1 g) of a moulded substrate is moved between a humidity generator, where it is conditioned to a certain water activity, and a microcalorimeter, where the heat production rate is measured. This is repeated for different water activities. A conditioning and the subsequent thermal measurement takes approximately one day for each water activity. Results are presented from a measurement with Penicillium mould growing on a bread substrate. The results correlate well with literature data indicating that the present method is a rapid way of assessing mould activity as a function of water activity.