Work shown in this paper reports measurements of fluid behavior and its dependence upon concentration in natural gas-like mixtures. These systematic experiments covering phase equilibrium and densities of selected mixtures use a state-of-the art apparatus. This work presents phase envelopes and density data for a synthetic natural gas-like mixture measured with an isochoric apparatus and a magnetic suspension densimeter. The isochoric apparatus permits precise, fast measurements using an automated procedure over wide ranges of pressure and temperature encompassing the cricondentherm and the cricondenbar. A comparison of the measured phase envelopes to predictions from cubic and molecular-based equation of states (EOS's) indicate serious deviations in both cricondentherm and cricondenbar regions. Although most design software use these models, the results obtained here indicate the EOS predictions are unacceptable. Current density measurements deviate from both the AGA8-DC92 and the GERG-2004 models.