Résumé : Abstract Background and Objectives Unprecedented social restrictions during the COVID-19 pandemic have provided a new lens for considering the inter-relationship between social isolation and loneliness in later life. We present these inter-relationships before and during the COVID-19 restrictions and investigate to what extent demographic, socio-economic, and health factors associated with such experiences differed during the pandemic. Research Design and Method We used data from four British longitudinal population-based studies (1946 NSHD, 1958 NCDS, 1970 BCS, and ELSA, N=12,129). Rates, co-occurrences, and correlates of social isolation and loneliness are presented prior to and during the early stage of the COVID-19 pandemic and the inter-relationships between these experiences are elucidated in both periods. Results Across the four studies, pre-pandemic proportions reporting social isolation ranged from 15 to 54%, with higher rates in older ages (e.g., 32% of 70-79 and 54% of those over 80). During the pandemic, the percentage of older people reporting both social isolation and loneliness and isolation only slightly increased. The inter-relationship between social isolation and loneliness did not change. Associations between socio-demographic and health characteristics and social isolation and loneliness also remained consistent, with greater burden among those with higher economic precarity (females, non-homeowners, unemployed, illness and greater financial stress). Discussion and Implications There were already large inequalities in experiences of social isolation and loneliness and the pandemic had a small impact on worsening extent and inequalities in these. The concepts of loneliness and social isolation are not interchangeable, and clarity is needed in how they are conceptualised, operationalised, and interpreted. Given many older adults experience high levels of social isolation, there should be greater emphasis on reducing social isolation and the inequalities observed in who experiences greater isolation and loneliness.