Thanks Rob. Ok, so it is allowed to use one side “liquid” (in my case, oxygen) and the other side “mixture” (in my case, dissolved oxygen and water).

In this case, if I use UDF (DEFINE_MASS_TRANSFER (name, c, mixture_thread, from_phase_index, from_species_
index, to_phase_index, to_species_index)) for interficial mass transfer, Fluent will pass -1 to “from_species_index” to represent the “liquid”. If I understand correctly?