Photo-oxidation of large malonamides N,N′-dimethyl, N,N′-dibutyl, tetradecyl malonamide (DMDBTDMA) and N,N′-dimethyl, N,N′-dibutyl, dodecylethoxy malonamide (DMDBDDEMA)—potential extractants for minor actinides in nuclear fuel reprocessing—leads mainly to the corresponding monoamides: N-butyl N-methyl hexadecanamide (BMHDA) and N-butyl N-methyl dodecyl oxy butanamide (BMDDOBA), respectively. The evolution of these main photoproducts was monitored during the degradation of malonamide solutions, and also pure monoamide solutions. Irradiation, for 1 h, with a low mercury pressure lamp can destroy BMHDA when diluted to 0.075 mol l−1 in n-dodecane. The two monoamides only differ by an ether bond and the weakness introduced by such a function in an alkyl chain is shown by slightly faster degradation. The relative weakness of BMDDOBA is independent of oxidant addition. Global degradation can be monitored by GC/FID or UV detection at 200 nm.