Abstract: A monolithic metal-organic framework (MOF) composite body is disclosed, comprising: MOF crystallites adhered to each other via a binder comprising MOF; and at least 0.15 vol % nanoparticles encapsulated in the MOF body. The nanoparticles have an average particle size corresponding to an average particle diameter in the range 3-200 nm. The nanoparticles may have photocatalytic activity. The MOF composite body is of use for treating water containing an organic dye, the photocatalytic reaction supported by the photocatalytic nanoparticles being a degradation reaction of the organic dye.