Research on private regulation of labour standards in global production networks often highlights their continuing failure despite the fact that lead firms no longer consider them as mere window dressing. Fewer analyses delve into their on-the-ground effectiveness to benefit workers. This article joins a context-specific approach with quantitative analysis to examine whether labour standards used in private regulation improve employment relationships in suppliers of global production networks. Based on a single-country case study of Brazil, we look at the extent of their adoption by suppliers across sectors, their complementarity with national labour institutions, and whether the adoption of labour standards at supplier site level is likely to support labour agency. Our findings show little effectiveness of labour standards against those dimensions. The presence of labour standards at supplier level alone has no significant impact and varies greatly across sectors. It is only if workers are aware of the presence of such standards that it might support their agency when union membership is taken as proxy. Yet, the correlation could also be the other way round: awareness of labour standards depend on being a member of a union in the first place.