Relative difference set Semi-regular relative difference set Motivated by a connection between semi-regular relative difference sets and mutually unbiased bases, we study relative difference sets with parameters (m, n, m, m/n) in groups of non-prime-power orders. Let p be an odd prime. We prove that there does not exist a (2p, p, 2p, 2) relative difference set in any group of order 2p 2 , and an abelian (4p, p, 4p, 4) relative difference set can only exist in the group Z 2 2 × Z 2 3 . On the other hand, we construct a family of non-abelian relative difference sets with parameters (4q, q, 4q, 4), where q is an odd prime power greater than 9 and q ≡ 1 (mod 4). When q = p is a prime, p > 9, and p ≡ 1 (mod 4), the (4p, p, 4p, 4) non-abelian relative difference sets constructed here are genuinely non-abelian in the sense that there does not exist an abelian relative difference set with the same parameters.