Greenhouse Gas Removal Techniques (GGR) appear to offer hopes of balancing limited global carbon budgets by removing substantial amounts of greenhouse gases from the atmosphere later this century. This hope rests on an assumption that GGR will largely supplement emissions reduction. The paper reviews the expectations of GGR implied by integrated assessment modelling, categorises ways in which delivery or promises of GGR might instead deter or delay emissions reduction, and offers a preliminary estimate of the possible extent of three such forms of ‘mitigation deterrence’. Type 1 is described as ‘substitution and failure’: an estimated 50-229 Gt-C (or 70% of expected GGR) may substitute for emissions otherwise reduced, yet may not be delivered (as a result of political, economic or technical shortcomings, or subsequent leakage or diversion of captured carbon into short-term utilization). Type 2, described as ‘rebounds’, encompasses rebounds, multipliers and side-effects, such as those arising from land-use change, or use of captured CO2 in enhanced oil recovery. A partial estimate suggests that this could add 25-134Gt-C to unabated emissions. Type 3, described as ‘imagined offsets’, is estimated to affect 17-27% of the emissions reductions required, reducing abatement by a further 182-297 Gt-C. The combined effect of these unanticipated net additions of CO2 to the atmosphere is equivalent to an additional temperature rise of up to 1.4°C. The paper concludes that such a risk merits further deeper analysis and serious consideration of measures which might limit the occurrence and extent of mitigation deterrence.