The size of fluidized agglomerates of nanoparticles was successfully measured in situ in conventional and microjet assisted gas fluidized beds by using Lasentec Focused Beam Reflectance Method (FBRM) and Particle Vision Measurement (PVM) probes In situ particle size distributions and agglomerate images of Aerosil R974 and Acrosil 90 nanopowders were obtained This was achieved by reducing the electrostatic charge in the fluidized bed by bubbling the gas through an alcohol water solution before entering the bed. Failure to remove electrostatic charges resulted in blocking of the probe lenses and blurred images or spiky size distributions The agglomerate size distributions show that Aerosil R974 agglomerates (APF-type nanopowder) are smaller and less dense than Aerosil 90 agglomerates (ABF-type nanopowder). These observations match their respective fluidization behavior and confirm that the APF-ABF classification is dependent on the size and density of the agglomerates The application of the microjet assisting method results in a further reduction in the mean size and density of the agglomerates and explains the higher quality of fluidization and larger bed expansion observed with the microjet