A closure procedure based on the probability density functions of the concentrations of chemically inert species has been proposed and applied to derive the mathematical forms of the local average values of the nonlinear reaction, nucleation and crystal growth terms. Applied to experimental results for BaSO4 precipitation, satisfactory agreement between predicted and observed results was obtained for two tubular reactors. In experiments solutions of BaCl2 and Na2SO4 were contacted in tubular reactor equipped with an impingement mix-head.