We discuss logarithmic corrections to form factors of mesons built from heavy quarks. The reactions e+e- → ηcγ and H → Jψγ are considered as an example. A novel feature as compared to the well-studied problem of the pion form factor is the existence of transitions between the quark-antiquark state cc and the gluonic one. O(αs) corrections are calculated exactly. An infinite series of the leading logarithmic terms (αsln[Q2/mc2])n is summed up with the help of the operator technique. Apart from ree results already known for quark operators, we use some new results referring to gluon operators and their mixing with those made from quarks. Two alternative derivations of the multiplicatively renormalizable operators are given. The first one reduces to a direct computation of the mixing matrix and its diagonalization, the second is based on conformal symmetry considerations.