This paper describes the development and preliminary psychometric evaluation of an instrument that measures the frequency of adaptive behaviours and cognitions related to therapeutic change during cognitive behavioural therapy (CBT), for symptoms of anxiety and depression. Two studies were conducted. In study one, 661 participants completed an online survey with 28 items targeting adaptive behaviours and cognitions. Exploratory factor analysis performed on part of the sample (n = 451) revealed that a four-factor solution ‘characterised’ the data. This led to the development of a 12-item instrument, the Frequency of Actions and Thoughts Scale (FATS). Confirmatory factor analysis was used to confirm the factor structure of the FATS using the remaining sample (n = 210), which revealed an acceptable model fit. In study two, 125 participants with clinically significant symptoms of anxiety, depression, or both were recruited to an Internet-delivered CBT (iCBT) treatment course. Participants completed the FATS and other measures throughout treatment, after treatment, and at three-month follow-up. Correlations and residual change scores of the FATS and its subscales with measures of anxiety, depression, behavioural activation, and CBT-related skills usage supported the construct validity of the FATS. A significant increase in FATS scores over treatment was also observed. The findings provide preliminary support for the psychometric properties of the FATS, which appears to have utility in research investigating mechanisms of change in CBT.