Abstract The turbulent diffusion of small, spherical particles suspended in a round, free jet is studied theoretically in the jet similarity region, with minimum recourse to the usual phenomenological arguments. Particles of infinitesimal size are treated by applying Taylor's theory of diffusion by continuous movements to define the fluid diffusivity of a “scaled” jet. The hypothesis is made that a transformation from Lagrangian to Eulerian variables, which is correct for homogeneous turbulence, may also be used in the similarity region of the jet. On this basis, the density distribution is obtained and found to agree quite well with experimental measurements of heat and trace gas diffusion. Particles of finite size are treated by a perturbation method, in which the infinitesimal particle result is invoked as a zero-order solution. The results are compared with available measurements of finite particle diffusion jets. Information on the radial spread of fluid and finite particles downstream of the jet orifice is also obtained.