Iapetus has been called the yin and yang of the Saturn moons because its leading hemisphere has a reflectivity (or albedo) as dark as coal (albedo 0.03-0.05 with a slight reddish tinge) and its trailing hemisphere is much brighter at 0.5-0.6.

Saturn's third largest moon, Iapetus has a mean radius of 457 miles (736 km) and a density only 1.2 times that of liquid water. It has been suggested that Iapetus (like Rhea) is three quarters ice and one quarter rock.

Iapetus orbits at 2,213,000 miles (3,561,000 km) from Saturn. The great distance from Saturn's tidal forces and from most of the other moons and ring particles has probably allowed the Iapetus surface to be largely unaffected by any melting episodes that could have caused some smoothing or "resurfacing" as on some of the moons closer to Saturn.

However, despite the great distance, Saturn has tidally locked Iapetus. The moon always presents the same face toward Saturn. With its distant, inclined orbit, Iapetus is the only large moon from which there is a nice view of the rings of Saturn.

As with some other Saturnian moons, Iapetus is in resonance with Saturn's largest moon, Titan, which orbits at 759,200 miles (1,221,850 km). That means that the two objects speed up and slow down as they pass each other in a complex set of variations. However, Iapetus has a diameter less than a third of Titan's diameter, so Titan's rotation and orbit are affected much less than those of Iapetus.