Background: Mental health problems are common and place a burden on the individual as well as on societal resources. Despite the existence of evidence-based treatments, access to treatment is often prevented or delayed due to insufficient health care resources. Effective internet-based self-help interventions have the potential to reduce the risk for mental health problems, to successfully bridge waiting time for face-to-face treatment and to address inequities in access. However, little is known about the cost-effectiveness of such interventions. This paper describes the study protocol for the economic evaluation of the studies that form the ICare programme of internet-based interventions for the prevention and treatment of a range of mental health problems. Methods: An overarching work package within the ICare programme was developed to assess the cost-effectiveness of the internet-based interventions alongside the clinical trials. There are two underlying tasks in the ICare economic evaluation. First, to develop schedules that generate equivalent and comparable information on use of services and supports across seven countries taking part in clinical trials of different interventions and second, to estimate unit costs for each service and support used. From these data the cost per person will be estimated by multiplying each participant's use of each service by the unit cost for that service. Additionally, productivity losses will be estimated. This individual level of cost data matches the level of outcome data used in the clinical trials. Following the analyses of service use and costs data, joint analysis of costs and outcomes will be undertaken to provide findings on the relative cost-effectiveness of the interventions, taking both a public sector and a societal perspective. These analyses use a well-established framework, the Production of Welfare approach, and standard methods and techniques underpinned by economic theory. Discussion/conclusion: Existing research tends to support the effectiveness of internet-based interventions, but there is little information on their cost-effectiveness compared to ‘treatment as usual’. The economic evaluation of ICare interventions will add considerably to this evidence base.