We report the discovery of two very cold and massive molecular cloud cores in the region ISOSS J18364-0221. The object has been identified by a systematic search for very early evolutionary stages of high-mass stars using the 170 micron ISOPHOT Serendipity Survey (ISOSS). Submm continuum and molecular line measurements reveal two compact cores within this region. The first core has a temperature of 16.5 K, shows signs of ongoing infall and outflows, has no NIR or MIR counterpart and is massive

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... nough (M ~ 75 M_sun) to form at least one O star with an associated cluster. It is therefore considered a candidate for a genuine high-mass protostar and a high-mass analog to the Class 0 objects. The second core has an average gas and dust temperature of only ~ 12 K and a mass of M ~ 280 M_sun. Its temperature and level of turbulence are below the values found for massive cores so far and are suggested to represent the initial conditions from which high-mass star formation occurs.