Abstract: Polyoxometalate (POM) based gels are synthesized by reacting POMs with an excess of caprolactam. These POM-entrapped caprolactam gels are thermoreversible and possess good mechanical strength. The gelation of POMs in caprolactam matrix brought the gel acidity similar to the physiological pH. The cytotoxicity studies on SCC 131 cell lines using these gels show 50% of cell death in the range of 2 to 6 mM concentration in 48 h. The supramolecular caprolactam matrix with active protons and the redox property of POMs are responsible for the cytotoxicity effect. Graphical Abstract: SYNOPSIS Two thermoreversible, hydrophilic gels containing Keggin-type polyoxometalates are synthesized by gradually increasing the caprolactam to polyoxometalate mole ratio. They both have physiological pH and show cytotoxicity effect on SCC 131 head and neck cancer cell lines within 24 hours.[Figure not available: see fulltext.] © 2018, Indian Academy of Sciences.