The present work investigates TBPB(tetra-n-butylphosphonium bromide) hydrate slurries based on focused beam reflectance measurement (FBRM) method. Hydrates can be used as phase change materials (PCM) for thermal energy storage due to their high latent heat of melting. In this study, hydrates were formed from TBPB-water mixtures, suitable for air-conditioning applications. Moreover, hydrate slurries, composed of hydrate-particles dispersed in liquid, are appropriate solutions for cold distribution. Nevertheless, flow properties of hydrate slurries, depending on hydrate crystallization kinetics, must be controlled. In this work, FBRM was used for analysing TBPB-hydrate-particle size distribution. This work showed that FBRM can detect system phase changes (nucleation, growth, agglomeration). In addition, a bimodal distribution of particles was observed. This work could be useful to understand rheological behaviour of hydrate slurry, not only for secondary refrigeration applications, but also for natural gas hydrate transportation, pipeline plugging prevention, and gas capture.