Zwolsman competed in the World Sportscar Championship in 1991 finishing 6th overall in the driver standings, running his own team (Euroracing) with a chassis from Spice Engineering. He attempted the series again in 1992 fielding a &lt;a href="Lola%20T92/10"&gt;Lola T92/10&lt;/a&gt;, but failed to score a point. However his Euroracing team finished 5th in the championship. The series folded the next year. He competed from 1990 to 1992 in the 24 Hours of Le Mans, having his best result in 1992 when he teamed with Heinz-Harald Frentzen and Shunji Kasuya to finish 13th. In 1992 he also competed in the Interserie, winning one race and finishing 10th in the drivers' standings. Zwolsman's son Charles Jr. would follow his father into motorsports, also competing in Le Mans as well as Formula Three and Champ Car.