A modified Leung-Griffiths correlation for the near-critical phase boundary of ammonia-water is extrapolated to low pressure. The extrapolation is based on the observation that, on a plot of logarithm of pressure vs. inverse temperature, dew and bubble isopleths form straight lines below the critical region. The extrapolation is performed both directly and with additional optimization to a chosen low-pressure data set. On the bubble side, the method yields a useful baseline calculation against which different experimental measurements can be tested for mutual consistency, but the approach is considerably less successful on the dew side. Published by Elsevier Science B.V.