The use of environmental offsets has increased in many parts of the world over the last decade, but assessments of their effectiveness have been rare. We provide the first quantification of the effectiveness of offsets in an Australian State (Western Australia) with an offsets register and history of offset implementation. We determined what outcomes were achieved and the environmental effectiveness of 208 past and current offsets applied as part of environmental approvals between 2004 and 2015 under State jurisdiction legislation. Of the past offsets, we conclude that at most 39% of the offsets studied delivered an outcome and can be considered effective, with land acquisition comparing favourably to other offset types. The outcomes of many offsets were unknown due to reporting too soon after implementation (14%) and inadequate reporting (18%). Thirty percent of past offsets during this time period were found be ineffective through non- or inadequate implementation. We observed significant improvements in the clarity of offset approval conditions over the time period of our study, nonetheless, we suggest that these results provide evidence of the need for better implementation of on-ground management and research into the nature of offsets. We make four suggestions for improvement: 1) timely reporting and compliance with environmental conditions; 2) ensuring approval conditions measure ecological outcomes; 3) improving project planning for offsets; and 4) including contingency and longer term planning in offset design. Conclusions from examining implementation of offsets in Western Australia are likely to be applicable wherever offsets policies are in place or being developed. © 2016 Elsevier Ltd.