A mesoporous organic material which makes it possible to extract, using the liquid-solid extraction technique, the uranium(VI) contained in an aqueous medium including phosphoric acid, with high efficiency and high selectivity for the iron that the medium can likewise contain. The material is likely to be obtained by cross-linking polymerisation of a monomer of formula (I) below, wherein: R1, R2 and R3 are, independently from one another, H, a C1 to C12 saturated or unsaturated, linear or branched hydrocarbon group, or a polymerisable group, with the condition that at least one of R1, R2 and R3 is a polymerisable group; R4 and R5 are, independently from one another, H or a C1 to C8 saturated or unsaturated, linear or branched hydrocarbon group; the cross-linking polymerisation being carried out in the presence of a cross-linking agent and one or more pore-forming agents.