The Mn-4 complex which is involved in water oxidation in photosystem II is known to exhibit three types of EPR signals in the S-2 state, one of the five redox states of the enzyme cycle : a multiline signal (spin 1/2) signals at g > 5 (spin 5/2), and a signal at g = 4.1 (spin value 3/2 or 5/2). The multiline and g = 4.1 signals are those the most readily observed. The relative proportions of the g = 4.1 signal and of the multiline signal are affected by many biochemical treatments including the substitution of Ca2+ and Cl- which are two essential cofactors for O-2 evolution. The state responsible for the multiline signal can also be converted, reversibly, to that responsible for the g = 4.1 signal upon the absorption of near-IR light at around 150 K. These infrared-induced effects are confined to the Mn4 cluster, and no other redox change occurs in the enzyme. Here, we have used the IR-induced photochemistry of the Mn4 cluster to measure the changes in magnetization occurring upon interconversion of the state responsible for the spin 1/2 state and the g = 4.1 state. Measurements were performed with a SQUID magnetometer below 20 K and at magnetic fields less than or equal to 5.5 T. Simulations of experimental data provide strong indication that the spin value of the state responsible for the g = 4.1 state is 5/2. Results are : discussed in terms of a model implying an IR-triggered spin conversion of the Mn-III (from the spin 2 to spin 1) of the Mn-4 cluster.