Some organisations still pay inconsistent rates to men and women doing the same job - according to a Institute for Employment Studies' (IES) report.
Equal pay reviews in practice, published as part of the Equal Opportunities Commission working paper series, used intensive case studies of 15 medium and large employers from across the UK. All the workforces comprised at least 20% women.
Nine of the 15 organisations recorded equal pay gaps, with the majority in favour of men, although a few favoured women.
Gaps ranged from men being paid 40% more than women to women being paid 14% more than men, although most discrepancies were between 2% and 15%.
The survey encouraged firms to carry out consistent methodology but there was considerable variation.
Reasons for unequal pay included line management discretion, market-related pay differences, shorter lengths of service, and overtime pay.
Most participants intend to address the problem although two employers saw no reason for action. Some 93% intended to undertake further equal pay reviews. Over half indicated the EPR had a positive effect on reducing gender pay gaps.
The Equal Pay Act of 1970 gives employees a right to the same pay and benefits as a person of opposite gender in the same employment.
Contact the EOC helpline on 0845 601 590 or view report online at www.eoc.org.uk/research