We present new data taken at 850 $mu$m with SCUBA at the JCMT for a sample of 19 luminous infrared galaxies. Fourteen galaxies were detected. We have used these data, together with fluxes at 25, 60 and 100 $mu$m from IRAS, to model the dust emission. We find that the emission from most galaxies can be described by an optically thin, single temperature dust model with an exponent of the dust extinction coefficient ($k_lambda propto lambda^{-beta}$) of $beta simeq 1.5 - 2$. A lower $betasimeq 1$ is required to model the dust emission from two of the galaxies, Arp 220 and NGC 4418. We discuss various possibilities for this difference and conclude that the most likely is a high dust opacity. In addition, we compare the molecular gas mass derived from the dust emission, $M_{dust}$, with the molecular gas mass derived from the CO emission, $M_{CO}$, and find that $M_{CO}$ is on average a factor 3 higher than $M_{dust}$.
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