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Experimental data for the magnetic susceptibility and magnetostriction of CuGeO 3 are analyzed within a one-dimensional antiferromagnetic model with nearest (J 1 ) and next-nearest neighbour interactions (J 2 ). We show that the ratio of the exchange constants in the antiferromagnetic chains of CuGeO 3 amounts to J 2 /J 1 = 0.354(0.01), i.e. it is significantly larger than the critical value for the formation of a spontaneous gap in the magnetic excitation spectrum without lattice dimerization. The susceptibility data are reproduced by our numerical results over the temperature range from 20K to 950K to a high degree of accuracy for J 1 = 80.2(3.0) and J 2 = 28.4(1.8). The pressure dependence of the exchange constants is estimated from magnetostriction data. Furthermore, the specific heat data are checked on consistency against the calculated entropy of the above model.