The thermal decomposition of LnA(3) (Ln is Ce, Pr, Nd, Er; A is [CH3CH(CH2CH3)(CH2)(4)O]2PO2-)(2) has been studied. It is established that heating to 443-453 K leads to recrystallization of the LnA(3) complexes accompanied by a partial annealing of the crystal defects. Their decompositions start above these temperatures and are complete at 583-608 K, producing Ln(PO3)(3). The decomposition process proceeds in a number of stages, namely the step-wise release of 6 mol of CH3, 6 mol of C3H6 and 3 mol of C8H16O, followed by a crystallization of the metaphosphates formed. Their activation energy depends on the nature of the lanthanon and (mainly for the first decomposition stages) on the heating atmosphere. Although a strong dependence is not apparent, it is clear that these factors are of prime importance for the determination of the rate-controlling processes.