The authors establish a topological version of a classical theorem proved by H. J. Keisler. In particular, it is shown that under the continuum hypothesis an \(L_t\)-sentence is preserved under reduced products of topological spaces if and only if it is equivalent in basic structures to an \(L_2\)-Horn sentence. That is to say that each \(L_t\)-Horn sentence is preserved under such products. The language \(L_t\) is a sublanguage of the monadic second-order language \(L_2\) that (using weak structures as models) can be regarded as a two-sorted first-order language.