  We find the cross-over behavior for the spin-spin correlation function for
the 2D Ising and 3-states Potts model with random bonds at the critical point.
The procedure employed is the renormalisation approach of the perturbation
series around the conformal field theories representing the pure models. We
obtain a crossover in the amplitude for the correlation function for the Ising
model which doesn't change the critical exponent, and a shift in the critical
exponent produced by randomness in the case of the Potts model. A comparison
with numerical data is discussed briefly.
