Four new compounds based on guanidinium and halogenoplumbate(II) ions, [C(NH2)3]2PbCl4, [C(NH2)3]2PbBr4, [C(NH2)3]2PbI4 and C(NH2)3PbI3, have been studied by differential thermal analysis and X-ray powder diffraction. In the chloride, four thermal anomalies have been detected at 345, 383, 412–419 and 439 K, indicating the possibility of four first-order phase transitions; the estimated transition entropies suggest the existence of phase disordering above 439 K. It was established that the phase sequences occurring on heating and on cooling the substance are different. The bromide compound undergoes two successive first-order phase transitions at 406–415 and 426 K, both associated with a large transition entropy indicating disordering of the intermediate- and high-temperature phases. In the tetraiodide, a continuous phase transition to the disordered phase was found at 307 K, while the triiodide undergoes two first-order phase transitions at 255 and 432 K, respectively. Both phase transitions in C(NH2)3PbI3 are of the order-disorder type, the high-temperature phase exhibits metastable properties.The spectroscopic changes observed at the high-temperature phase transitions in [C(NH2)3]2PbBr4 and C(NH2)3PbI3 indicate that the mechanism of these transformations is connected both with a disordering process and a distortion of the anionic sublattices.