  The anomalous dimension matrix of dimensionally regularized four-quark
operators is known to be affected by evanescent operators, which vanish in
$D=4$ dimensions. Their definition, however, is not unique, as one can always
redefine them by adding a term proportional to $(D-4)$ times a physical
operator. In the present paper we compare different definitions used in the
literature and find that they correspond to different renormalization schemes
in the physical operator basis. The scheme transformation formulae for the
Wilson coefficients and the anomalous dimension matrix are derived in the
next-to-leading order. We further investigate the proper treatment of
evanescent operators in processes appearing at second order in the effective
four-fermion interaction such as particle-antiparticle mixing, rare hadron
decays or inclusive decays.
