In a sample of 1449 adults, divided by age into young, middle-age, and old, mood was found to consist of two age invariant components, vigour and affect. Factor structure differed by age for affect but not vigour. For old adults, two unipolar affect factors were obtained, whereas one bipolar affect factor was obtained for the two younger groups. From these factor analyses a mood scale (the Memorial University Mood Scale, the MUMS) was developed and its predictive validity and reliability established for all age groups. Using the MUMS, mean level differences by age were found in both vigour and affect, as well as a measure of globed mood, with the old adults higher on all three measures. Age invariant, diurnal patterns were found for both vigour and affect. Vigour followed an inverted U-shaped diurnal pattern and affect a primarily linear pattern, suggestive of appraisals of somatic state and environmental conditions, respectively.