Positron emission tomography (PET) is a powerful molecular imaging technique based on the administration and detection of radioactive (positron emitting) species. In some applications, the concept of specific activity becomes especially important in order to prevent undesired pharmacological and/or toxic effects after injection of the radiotracer. Problems to obtain high specific activities are found when (11)C-labeled compounds are prepared by methylation following the so called "wet" method, which consists of a simple route but usually yields radiotracers highly diluted with the stable specie. In the present work, the main sources of contamination by stable carbon in the [(11)C]CH(3)I synthesis following the "wet" method have been analyzed and their individual contribution has been quantified. The results show that the most relevant contamination of CO(2) is generated during the bombardment process.