EN
We prove new necessary and sufficient conditions for a morphism of coalgebras to be a monomorphism, different from the ones already available in the literature. More precisely, φ: C → D is a monomorphism of coalgebras if and only if the first cohomology groups of the coalgebras C and D coincide if and only if $∑_{i∈I} ε(a^{i}) b^{i} = ∑_{i∈I} a^{i} ε(b^{i})$ for all $∑_{i∈I} a^{i}⊗ b^{i} ∈ C ◻_{D} C$. In particular, necessary and sufficient conditions for a Hopf algebra map to be a monomorphism are given.