The mixing characteristics, flow patterns and hydrodynamics of gas-solid spouted beds depend on the type of operating flow regime. In the present work, Gamma-Ray Densitometry (GRD) technique has been developed and implemented for the first time to identify different flow regimes and their transition velocities in gas-solid spouted beds. Two spouted beds (0.152 m and 0.076 m diameter) have been used to identify various flow regimes. Three statistical parameters (mean, variance and flow regime indicator (I)) have been applied to analyze the photon counts from GRD. It was found that the implementation of such non-invasive radioisotope based technique, GRD, was successfully able to identify different flow regimes and their transition velocities in gas-solid spouted beds. Flow regimes of packed bed, stable spouting and unstable spouting have been identified in the two studied spouted beds. Furthermore, the stable spouting regime achieved at larger magnitudes of the range of the superficial gas velocity in a larger diameter bed as compared to those in a smaller diameter bed.