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We discuss the sign problem arising in Monte Carlo simulations of frustrated quantum spin systems. We show that for a class of “semifrustrated” systems [Heisenberg models with ferromagnetic couplings J z (r)< 0 along the z axis and antiferromagnetic couplings J xy (r)=− J z (r) in the xy plane, for arbitrary distances r], the sign problem present for algorithms operating in the z basis can be solved within a recent “operator-loop” formulation of the stochastic series expansion method [a cluster algorithm for sampling the diagonal matrix elements of the power series expansion of exp (− β H) to all orders]. The solution relies on the identification of operator loops which change the configuration sign when updated (“merons”) and is similar to the meron-cluster algorithm recently proposed by Chandrasekharan and Wiese for solving the sign problem for a class of fermion models [Phys. Rev. Lett. 83, 3116 (1999)]. Some …