The knowledge of optimum plate spacing between two heated vertical plates is essential for the design of compact and thermally efficient electronic packages. A discrete/continuous system model was developed and the governing equations were solved using an implicit finite difference technique to determine the optimum plate spacing ((B/L)opt). In this work consideration was given to both uniform wall temperature (UWT) and uniform heat flux (UHF) conditions. Calculations were made for a Grashof number (Gr) range of 10 - 105, and for asymmetric heating parameter (γ) values of 0., 0.1, 0.5 and 1.0. At large values of (B/L) the left plate average channel Nusselt number approaches the single plate limit. As (B/L) decreases the channel average Nusselt number increases due to the 'chimney' effect and eventually decreases to the fully developed limit. The decrease in the channel average Nusselt number on the lower side of (B/L)opt is steep compared to the decrease on the higher side of (B/L)opt. The impact of the asymmetric heating parameter on (B/L)opt was found to be minimal.