We measured natural ortho-para conversion rates within a wide region o
f hydrogen fluid stares: at temperatures 17-32 K in the liquid ann at
temperatures 40-120 K in the gas for densities trp to 0.09 g/cm(3). Th
e experimental data for the gas phase ale interpreted within the frame
work of a theory, the basic distinction of which from Wigner's approa
ch is that we take into account the dependence of the closest distance
in a collision of two orthomolecules on their velocity (i.e., tempera
ture). Our theory yields results in good qualitative agreement with ex
periment in the gas phase. In order to describe the entire bulk of con
version rate data in the hydrogen fluid phases we suggest a convenient
interpolation formula, which has reasonable physcial grounds.