The terrorist attacks on the United States in September of 2001 had a deep impact on the world. In the aftermath of the attacks strong discursive and legislative measures were taken against terrorism. One of these measures was to proscribe some armed groups on so-called terrorist lists, with subsequent sanctions inflicted on these groups. In this thesis I aim to understand the impact the mechanism of proscription can have on a peace process where one of the parties in the process is proscribed on one or more of these lists. To further my understanding of this impact I have chosen to analyze the case of the 5th Sri Lankan peace process. The armed group the Liberation Tigers of Tamil Eelam (LTTE) were engaged in a decades-long conflict with the Sri Lankan state with the goal of creating their own state for the Tamil people of Sri Lanka. The LTTE and the government of Sri Lanka became involved in a peace process lasting from the late 1990s until the destruction of the LTTE in 2009, with the formal talks of the process occuring in 2002-2003. This process was facilitated by Norway, in addition to other less involved international stake-holders. In the thesis I analyze the impact the terrorist proscriptions of the LTTE had on the peace process, and to what degree this impact can help explain the failure of the process. It is important to understand the consequences of the proscriptions, as my analysis indicates that the proscriptions of the LTTE were partially to blame for the failure of the process. The proscriptions prevented contact between crucial actors such as the US and the EU to the LTTE, in addition to decreasing the levels of trust in the process. This reduced the ability of the third-party actors to affect the strategic calculations of the LTTE and move the group away from using violent means. The proscriptions thus seem to have had a negative effect on the framework for the peace process. In addition to the effect on the framework of the process, I analyzed whether the threaths and promises of proscription and de-proscription were effective in altering the incentive-structure of the LTTE. My analysis found that the promise of de-proscription from the Sri Lankan government was effective in getting the LTTE to hold formal talks. The international actors threaths and promises on the other hand were ineffective in their aim of moving the LTTE away from violent means. The proscriptions were thus found to have had a negative impact on the peace process, with no real positive effect to compensate for this.