Given a sequence of integers a j , j 1, a multiset is a combinatorial object composed of unordered components, such that there are exactly a j one-component multisets of size j. When a j j r−1 y j for some r > 0, y 1, then the multiset is called expansive. Let cn be the number of multisets of total size n. Using a probabilistic approach, we prove for expansive multisets that cn/c n+1 → 1 and that cn/c n+1 > 1 for large enough n. This allows us to prove monadic second-order limit laws for expansive multisets. The above results are extended to a class of expansive multisets with oscillation.Moreover, under the condition a j = Kj r−1 y j + O(y νj ), where K > 0, r > 0, y > 1, ν ∈ (0, 1), we find an explicit asymptotic formula for cn. In a similar way we study the asymptotic behavior of selections, which are defined as combinatorial objects composed of unordered components of distinct sizes.