Alexithymia is a personality construct that manifests as difficulties with the cognitive and/or affective component of emotion processing. The present study examined two of the currently-debated theories regarding the neural basis of alexithymia: 1) that alexithymia is caused by a right hemisphere dysfunction of emotion processing areas, or 2) that alexithymia is caused by an interhemispheric transfer deficit. Participants with and without alexithymia, as classified by the Toronto Alexithymia Scale and the Bermond-Vorst Alexithymia Questionnaire, completed a computerised task in which consecutively appearing pairs of stimuli were presented in the left visual field, the right visual field, or in crossed visual fields. Participants determined whether the two stimuli were the same or different by pressing a button with the left or right hand. The stimuli included circles of varying sizes, or faces displaying varying emotions. Reaction time and accuracy were compared across alexithymia groups, with a focus on two key conditions which distinguished between the two theories. In the first key condition, stimuli were presented in the left visual field and a response was made with the left hand. In this condition, the right hemisphere was preferentially activated, meaning that if a right hemisphere dysfunction was present, then participants with alexithymia would perform more poorly in this condition than participants without alexithymia, especially in response to emotion stimuli. In the second key condition, the stimuli were presented in the right visual field and a response was made with the left hand. In this condition, stimuli were initially processed in the left hemisphere, and then a response was executed by activating the right hemisphere, meaning that interhemispheric communication was required. Thus, if an interhemispheric communication deficit was present, then participants with alexithymia would perform more poorly in this condition than participants without alexithymia. Overall, support was found for the Right Hemisphere Dysfunction theory when specifically examining the types of alexithymia that involve a deficit in affective processing, and support for the Interhemispheric Transfer Deficit theory was found when other classifications of alexithymia were used, particularly those that did not account for the affective component of alexithymia. These findings suggest that different types of alexithymia may have different underlying causes, and are discussed in the context of brain organization and functional laterality.