Let A and B be the generators of strongly continuous semigroups (S(t))t ≥ 0 and (T(t))t ≥ 0, respectively. Denote by Δ (t) = T(t) − S(t). We show that if Δ(t) is norm continuous for t > 0 and R(λ,B) − R(λ,A) is compact for λ ∈ ρ(A)⋂ ρ(B) , then Δ(t) is compact. The converse is true if the perturbing operator is of Miyadera-Voigt-type. A characterization of norm continuity of Δ(t) in terms of the resolvents of the generators is given in Hilbert spaces.