We show that an accurate description of the mass transfer controlled regime in long catalytic monolith channels in which the flow is fully developed and laminar, requires the knowledge of two constants, namely, the asymptotic Sherwood number (Sh(infinity), or dimensionless mass transfer coefficient) and the first normalized Fourier weight, alpha(1). While the first of these has received considerable attention in the literature, the second factor has been assumed to be unity. Because of this systematic error (of about 20% for common channel geometries), literature data and correlations for mass transfer in monoliths have large uncertainties. We present new experimental data that corroborates the theory and a new method for experimental estimation of the asymptotic constants for any arbitrary channel shape. For the case of a square channel we obtain experimental values for the constants of Sh(infinity) = 2.92 +/- 0.16 and alpha(1) = 0.78 +/- 0.09, which are in excellent agreement with the theoretical values (Sh(infinity) = 2.977 and alpha(1) = 0.8074). Using the asymptotic solutions of the convection-diffusion equation, we develop new correlations for mass transfer coefficients for the case of developing laminar flow that agree with numerical solutions of the Navier-Stokes equations, and theory in the limits. (C) 2003 Elsevier B.V. All rights reserved.