A growing body of evidence has suggested that patients suffering from the motor disorders Multiple Sclerosis (MS) and Parkinson’s disease (PD) also suffer from cognitive deficits. Although the prevalence of cognitive impairments in these patient groups is well documented there remains no explicit theory to explain the source of these impairments, with two main theories competing as an explanation; slowed information processing speed and executive dysfunction. The aim of the current study was to assess the specific cognitive impairments of both patient groups, and determine which competing theory was most relevant to each group. As participants within these patient groups are more susceptible to displaying mild to severe motor slowing or motor impairments, inspection time tasks as opposed to reaction time tasks were used, controlling for impaired motor responses. Eight MS patients, eight matched healthy controls and eight PD patients with eight matched controls were tested on a battery of neuropsychological tests to assess specific cognitive impairments. Three experimental tasks were used to directly assess the theories underlying the cause of any cognitive impairment found: a Pre-Loaded Dual Task (assessed executive dysfunction), VIT (information processing speed) and Letter RSVP (information processing speed). PD patients were not found to be impaired on any neuropsychological or experimental tasks when compared with their respective matched controls. MS patients were impaired on a number of neuropsychological tasks which directly assessed executive function; working memory, attention and planning. MS patients were significantly impaired on the Pre-Loaded Dual Task and showed a trend towards significance on the Letter RSVP task. It was argued that the cognitive impairments displayed by the MS patients were due to an executive dysfunction. However, as the study assessed small sample groups it was argued that cognitive impairments in MS patients may also be as a result of slowed information processing speed, combining both competing theories, although the current study did not have the statistical power to show a statistically significant difference in MS patient and control scores in tests of slowed information processing speed.