Trade unions’ capacity to engage in sustainability-related activities varies across European countries, yet comparative quantitative measures of these sustainability affordances remain limited. This data descriptor presents a dataset on the institutional, social and environmental determinants of trade union sustainability affordances in 20 European countries. The dataset was compiled from the Institutional Characteristics of Trade Unions, Wage Setting, State Intervention and Social Pacts (ICTWSS) Database and the International Social Survey Programme (ISSP) Environment modules (2010 and 2020), complemented by Organisation for Economic Co-operation and Development (OECD) data on trade union density and collective bargaining coverage. It comprises 29 indicators on collective bargaining structures, membership demographics and the environmental attitudes and behaviours of trade union members, which were normalised and aggregated into three subindices and a composite index grounded in affordance theory. The repository provides raw, processed and analysed data together with R scripts that allow the calculations to be replicated. The institutional and environmental subindices show good internal consistency (Cronbach’s alpha of 0.866 and 0.873). The dataset supports cross-country comparisons, research on organised labour in sustainability transitions and the evaluation of just transition policies.