The question is very non-specific.  Light hydrocarbons, thiols and sulfides come readily to mind.  Can you be more specific for the application?  Just for the sake of discussion, would methyl mercaptan be useful to you?  It is widely used as an odorant for propane because it will dissolve in the liquid and vaporize with the gas.

Hi Ray  ;

Thanks for your answer. my answer was really too broad .

I am looking for a solid-whether a blend or one substance -that will crack or dissolve in propane -  ideally in its vapour phase -  mixed with air.  I already found that resins - natural and artificial -based on terpenes -   are to same extent suitable.  I failed to find other materials .  i am very thankfull for your help

best  Yosi