We have used polarized light metallography to study magnetic domain configurations in Co5Sm, Co5Y, Co5Ce, and Co5Pr as a function of crystal thickness. From measurements of equilibrium domain widths on thin crystals, we estimate the domain-wall energy in these compounds to be 85, 35, 25, and 40 erg/cm2, respectively. From surface domain observations on bulk crystals, we have also made crude estimates of wall energy for Co5Nd, Co5La, and Co5Gd. From these wall energies and published anisotropy constants, we have calculated exchange constants, domain-wall thicknesses, and critical single-domain particle sizes. We find this critical particle size to be larger for Co5Sm and Co5Gd than for the other compounds, and suggest that this may partly explain why high coercive forces are more easily attained in powders of these two compounds.