The rims are 32 hole Mavic CXP-33 and have a road rim's typically narrow width of 19.4mm. Mavic very conservatively rates them for only up to 28mm tires, which is what I normally run on them. I know that cyclocross riders routinely run wider tires on them and the Contact 37s fit up with no problem, but I would be happier with something wider. I am thinking of building some wheels on wider rims, maybe Velocity Dyads with 32F/36R spokes for permanent multi-surface use.