The meanings of these words are so different that they should never be confused, but they often are. Involuntary means "unintentional" or "unconscious" (an involuntary movement of the eyes); "independent of one's will," "made or done other than by one's own choice" (an involuntary witness to the crime). Voluntary means "undertaken or brought about by one's own choice, one's free will (a voluntary contribution to the campaign); "according to one's own will or desires" (voluntary cooperation with the police). Voluntary implies the previous use of judgment and consideration (a voluntary admission of guilt); involuntary implies lack of conscious control and is the direct opposite of intentional (involuntary twitching of tired muscles).
Dictionary of problem words and expressions. Harry Shaw. 1975.