The temperature dependencies of a spontaneous polarisation, P_{s}, and pyroelectric modulus, γ, at different electric field intensities were investigated for the triglycine sulphate crystals doped with the Cr^{3+} chromium ions. On the base of thermodynamic theory the temperature dependencies of P_{s} and γ at different electric field intensities were estimated. The coefficients of the expansion of the thermodynamic potential were determined. The experimental results were compared with those obtained according to the thermodynamic theory.