Holding temperature and pressure constant, what is the most important feature in determining the phase of a given organic compound?

The type of intermolecular forces present.

Explanation:

Intermolecular forces are forces that hold a substance together in a particular star of matter. There are three states of matter; solid liquid and gas. What determines whether or not an organic substance will be found in a particular state of matter at constant temperature and pressure is the nature of intermolecular forces present.

For instance, alkanoic acids mostly exist as liquids due to intermolecular hydrogen bonding. The alkanes are mostly gases due to weak van der Waals forces present.