The influence of different fluid inlet types, slits or tubes, on mass transfer in a rectangular reactor was studied. Measurements of mass transfer coefficients were made using the limiting diffusion current technique based on ferricyanide ion reduction at a large nickel electrode located downstream of abrupt expansions. The overall mass transfer coefficients obtained were 3 to 13 times greater than those obtained in fully developed flows. Overall mass transfer coefficients were correlated for Reynolds numbers ranging from 400 to 3500 by a unique equation by introducing a nondimensional expansion factor defined by the ratio of the fluid inlet cross-section to that of the reactor. The correlation equation obtained was compared with literature data.