Microchannel blockage phenomena by hard, spherical particles have been investigated experimentally and theoretically. The study was performed over a range of particle-to-channel diameter ratios of 0.14 < R < 0.65. Two mechanisms have been investigated: orthokinetic flocculation and hydrokinetic arching. Arching appears to be the main mechanism for large, hard particles. In the absence of Brownian motion and inter-particle repulsive forces, other than simple Hertzian contact force, the blockage phenomenon is described by three non-dimensional parameters, N, R and β. The mean total number of particles in the channel having length L is N. Ratio of a diameter of particle (d p) and a diameter of channel (D) is R. Blockage efficiency factor β is determined experimentally. The data shows that a critical value N c exists as a function of R. N > N c implies high likelihood of blockage; if N < N c, blockages were never observed. The critical number decreases dramatically with increasing R. Blockages can occur at surprisingly low values of the volume concentration (φ). The experimental results matches well with the theory for the combinations of straight glass capillary, 76<D<156μm, 100mm-length, and spherical polymer particle, 22>d p<48μm.