The smectic A (SA)-chiral smectic C (S*C) transition in binary mixtures of methylchloropentanoyloxy heptyloxybiphenyl (C7) and butyloxyphenyl decyloxybenzoate (10O.4) changes from first order to second order as a function ofX, the mole per cent of 10O.4. High resolution heat capacity measurements show that the Cpbehaviour is well-described by Landau mean field theory. Classical tricritical behaviour is observed nearXtc≃ 10·6 and the coefficientbof the quadratic term in the free energy varies like (X - Xtc) in good agreement with results from polarization data.