The relationship between unions and earnings dispersion is examined using establishment-level data from the 1980, 1984 and 1990 Workplace Industrial Relations Surveys. Initially the cross-sectional relationship is examined using the 1990 data. The earnings dispersion of skilled and semiskilled workers is seen to be lower across unionized establishments than across non-union establishments; secondly, within-establishment earnings dispersion is lower in plants which recognize trade unions for collective bargaining purposes than in those that do not. All three surveys are then utilized to ascertain to what extent the decline in unionization in Britain has contributed to the rise in earnings inequality of semi-skilled workers. There was a sizeable and important widening of the gap in the dispersion of earnings across union and non-union plants between 1980 and 1990. For semi-skilled earnings, the decline in the share of plants with recognized unions can account for 11-17 percent of the rise in earnings inequality over this time period. The importance of falling union activity (as measured by union recognition) seemed to accelerate through the 1980s. Between 1980 and 1984 the relatively small falls in aggregate recognition explain less than 10 percent of the inequality increase, whereas between 1984 and 1990 about one-quarter of the increase can be accounted for by the fall in unionization. The majority of the rise in earnings inequality is, however, due to a large increase in earnings dispersion across non-union establishments.