ABSTRACT Individual level data is used to consider the effects of the union mark‐up on earnings in Northern Ireland and in particular to find out if a differential exists between bargaining groups as well as across covered and not covered workers. The results show that company (and to a lesser extent, UK industry‐wide) agreements have a much lower mark‐up than might be deduced by simply analysing mean wage levels across groups. This suggests that most of the differential is actually the result of a productivity advantage attached to (these) covered workers and not because of union power per se.