The T 2 relaxation time of protons in water suspensions of iron-oxide particle clusters is found to increase with the waiting time t w after the sample is inserted in the gap of the magnet with 1.41 T field, owing to field induced cluster aggregation. In order to compare the experimental results with existing theories, the measured T 2 has to be extrapolated to t w = 0 , for which a power-law T 2 increment versus t w relation is proposed. The extrapolated T 2 ( 0 ) is found to be inversely proportional to the volume fraction v of iron oxide.