We extend a lemma by Matsuda about the irreducibility of the binomial X π − 1 in the semigroup ring F [X; G], where F is a field, G is an abelian torsion-free group and π is an element of G of height (0, 0, 0, . . . ).In our extension, G is replaced by any submonoid of (Q + , +). The field F , however, has to be of characteristic 0. We give an application of our main result.