Summary: Let \(G\) be a solvable Lie group, \(\Gamma \subset G\) a lattice and \(S\subset \Gamma\) a semigroup. It is proved that \(S\) is a group provided it is not contained in a semigroup with non-empty interior of \(G\) and \(\Gamma\) satisfies a condition which is described by means of the complex weights of the adjoint representation of the Lie algebra of \(G\). The methods follow the same pattern as those developed by \textit{J. D. Lawson} [Proc. Edinb. Math. Soc., II. Ser. 30, 479-501 (1987; Zbl 0649.22004)] in the analysis of the semigroups with interior points in \(G\), and as such they require a machinery about semigroups in finitely generated groups.