Given a commutative ring R with 1 ∈ R and the multiplicative group R * of units, an element u ∈ R * is called an exceptional unit if 1 − u ∈ R * , i.e., if there is a u ∈ R * such that u + u = 1. We study the case R = Z n := Z/nZ of residue classes mod n and determine the number of representations of an arbitrary element c ∈ Z n as the sum of two exceptional units. As a consequence, we obtain the sumset Z * * n +Z * * n for all positive integers n, with Z * * n denoting the set of exceptional units of Z n .