Tradable green certificates (TGCs) have recently become a diffuse instrument to support renewable electricity in OECD countries. Although it is perhaps too early to draw a conclusive judgement on the effectiveness of this instrument in increasing renewable capacity and decreasing the price of certificates, one view in the literature maintains that long-term contracts are of particular importance for TGCs to be effective. This paper contributes to this debate by analysing how financial constraints and technological progress can induce investors to hold pessimistic expectations of their ability to sell green certificates and still make a profit. Clearly, these expectations will prevent investors from building new capacity to fulfil the quota comprised in TGCs and will keep the price of certificates traded in the market high. As this kind of expectation is not influenced by most design features of TGCs, one can conclude that long-term contracts are particularly important in determining the effectiveness and cost-effectiveness of these instruments. Some attention should therefore be paid to the features of the TGCs, which induce obliged parties to offer long-term contracts to renewable generators. (c) 2007 Elsevier Ltd. All rights reserved.