  Given a singular modulus $j_0$ and a set of rational primes $S$, we study the
problem of effectively determining the set of singular moduli $j$ such that
$j-j_0$ is an $S$-unit. For every $j_0 \neq 0$, we provide an effective way of
finding this set for infinitely many choices of $S$. The same is true if
$j_0=0$ and we assume the Generalized Riemann Hypothesis. Certain numerical
experiments will also lead to the formulation of a "uniformity conjecture" for
singular $S$-units.
