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The (100), simple-cubic, lattice-gas interface structure has been simulated by Monte Carlo methods. For the interface width $L$ we find $L=(1.05\ifmmode\pm\else\textpm\fi{}0.04){(1\ensuremath{-}\frac{T}{{T}_{c}})}^{\ensuremath{-}0.69}$, in the range $T\ensuremath{\gtrsim}0.5{T}_{c}$. This is larger than the bulk correlation length $\ensuremath{\xi}$ by a factor of \ensuremath{\sim} 3.

We have studied the interface between regions of opposite spin in the simple cubic Ising model. Low-temperature expansions of moments of the gradient of the density profile and of the slope at its midpoint suggest that the interface width diverges at a temperature ${T}_{R}$ about half the critical temperature. We discuss the physical significance of this transition and implications for general theories of interfacial properties.