The structure of QCD two-point correlation functions like the vector or scalar correlators at high orders in perturbation theory is studied. This is first exemplified within the large-beta_0 approximation, for which analytical results are available. In the case of full QCD, one has to resort to models for the Borel transform, though the general structure can be inferred from the renormalisation group. For this task, the C-scheme coupling a^ proves useful. It will also be discussed, how even-integer zeta-function values cancel in higher-order coefficients, if the C-scheme is employed.