Acute CrVI water pollution due to anthropogenic activities is an increasing worldwide concern. The high toxicity and mobility of CrVI makes necessary to develop dual adsorbent/ion-reductive materials able to capture CrVI and transform it efficiently to less hazardous CrIII. The accurate description of the chromium speciation at the adsorbent/ion-reductive matrix is key to assess whether CrVI is completely reduced to CrIII or if an incomplete transformation leds to the stabilization of highly reactive CrV transient species within the material. With this goal in mind, a dual ultraviolet visible and electron paramagnetic spectroscopy approach has been applied to determine the chromium speciation within Zr-based metal-organic frameworks (MOFs). Our findings point out that the generation of defect positions at Zr-MOFs boost CrVI adsorption, whilst the presence of reductive groups at the organic linkers play a key role to stabilize CrIII as isolated and/or clustered ions.