A one group, bare reactor diffusion theory model has been applied to data from the startup experiments performed on the Yankee reactor. This data includes differential banked control rod worths at various bank heights, the critical balance being maintained by varying concentrations of boric acid, and differential boron worth. The results show an excellent fit to the hot (514°F) data, yielding M2, k∞, keff, and the shutdown k with high precision. The 100°F data did not show such good agreement, probably due to the narrower range of boron concentrations which could be employed at that temperature.