Purpose: To investigate the x‐ray dose versus 1H NMR spin lattice relaxation (T1) response of ion exchange resin beads, with and without ferrous ions. Method and Materials: Strong acid cation exchange resin beads (Rohm and Haas, Amberlyst 15 Wet) were rinsed in distilled water and packed in plastic vials. Similar beads were treated to produce a 1 mM ferrous ion concentration inside the beads, then rinsed with distilled water, and finally packed in plastic vials. Sample vials were placed in a water phantom and exposed to 6 MV x‐rays from 0 to 100 Gy. The T1 for each sample was measured in a 1.5 Tesla MRI unit. Beads from two manufacturing lots, Lot 1 and 2, were tested. Density of the bead samples was 1.10 g/cm3. Results: Vials with beads containing ferrous ions exhibited a linear dose versus R1 (i.e., 1/T1) response with slopes of 0.102 and 0.093 sec −1 Gy−1 for respectively Lot 1 and Lot 2 beads. Vials of Lot 1 beads containing no ferrous ions exhibited a linear dose versus R1 response with a slope of 0.083 sec−1 Gy−1. Vials of Lot 2 beads containing no ferrous ions showed an exponential increase in R1 of 2.4 sec−1 from 0 to 50 Gy with minimal increase in R1 from 50 to 100 Gy. Conclusion: For both bead lots, the beads containing ferrous ions exhibited similar linear dose versus R1 responses. Beads containing no ferrous ions exhibited significant, but different dose versus R1 responses. When the ferrous ions are present in the beads, a ferrous oxidation reaction appears to dominate the dose response. The results support the concept of using strong acid cation exchange resin beads containing ferrous ions to create a three dimensional dosimeter which can be evaluated using MRI.