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In the present work the behaviour of cluster excitations in magnetic field, B, paralleł to the easy axis was experimentally studied by the measurements of ΚΕr(ΜοO 4)2 and CsDy(MoO 4) 2 specific heat, C, vs. B dependence studied at a constant temperature. Corresponding Monte Carlo simulations of C revealed that despite of the fact that the Ising exchange-coupling models proved useful for a sufficient description of the speciffc heat of these dipolar systems in B = 0, the analysis of current experimental data requires a partial renormalization of exchange-coupling constants previously obtained from B = 0 analysis. The influence of crystal field effects and long-range character of spin correlations is discussed.