Bis-triazinyl-pyridines have been developed at CEA-Marcoule to separate actinides(III) from lanthanides(III). Although high separation performances were observed in the counter-current hot test carried out in 1999 on a highly active effluent, the extraction and back-extraction yields of actinides(III) were lower than expected from calculation. A strong sensitivity of the extractant towards air oxidation and acidic hydrolysis was actually pointed out. Therefore, the hydrolysis of bis-triazinyl-pyridines has been deeply investigated (qualitative determination of the degradation products) in different experimental conditions. It appeared that the branching of the alkyl groups on the α position of the triazine rings hindered the hydrolysis of the extractant. The formulation of the solvent was thus optimized and tested again in 2001 on a genuine Diamex back-extraction solution in the Atalante facility.