Modelling Activities of Daily Living (ADLs) is an important step in the process to design and implement reliable sensor systems that effectively monitor the activities of the ageing population. Once modelled, unusual activities may be detected that have the potential of impacting upon a person’s well-being. The use of Petri nets to model ADLs is considered in this research as a means to capture the intricate behaviours of ambient systems. To our best knowledge there has not been extensive work in the related literature, hence the novelty of this work. The ADLs considered in the developed Petri net model are”:” (i) preparing tea, (ii) preparing coffee, and (iii) preparing pasta. The first two ADLs listed are deemed to have many occurrences during a typical day of an elderly person. The third activity is representative of activities that involve cooking. Hence, abnormal behaviour detected in the context of these activities can be an indicator of a progressive health problem or the occurrence of a hazardous incident. The completion and non- completion of activities are considered in the developed Petri net model and are also formally verified. The description of the sensor system of the kitchen ADLs, its Petri net model and verification results are presented. Results show that the Petri net modelling of ADLs can reliably and effectively reflect the real behaviour of the examined system detecting all the activities of the users that can exhibit both their normal and abnormal behaviour.