Okay Rob, let’s see if you can give some information about this one. In the manual, the gas phase species mass source term is defined as dzdt->species[0..], where I am guessing 0 is the species index (in this case, the first species in mixture materials). Then, I am expecting dzdt->species +=quantity to cause a change over species 0 concentration (which is not happening). In the manual there is an example of the usage of this structure: “dzdt->species[gas_index] += vap_rate;”, where gas_index is “gas_index = TP_COMPONENT_INDEX_I(tp,ns);”. I have read that TP_COMPONENT_INDEX_I(tp,ns) is supposed to return the gas index (if the component is not meant to evaporate, this gives -1). Could you clarify me what is the correct way to use this macro? Thank you!