The objective of the study has been to estimate the residual activity in the decommissioning waste of the TRIGA Mark II–type research reactor FiR 1 in Finland. Neutron flux distributions were calculated with the Monte Carlo code MCNP. These were used in the ORIGEN-S point-depletion code to calculate the neutron-induced activity of materials at different time points by modeling irradiation history and radioactive decay. Knowledge of the radioactive inventory of irradiated materials is important in the planning of the decommissioning activities and is essential for predicting the radiological impact to personnel and the environment. Decommissioning waste consists mainly of ordinary concrete, aluminum, steel, and graphite parts. Results include uncertainties due to assumptions on material compositions and lack of some detailed operational history data. Comparison to activity inventory estimates of two other decommissioned research reactors is also presented.