Taxation of mobility allowance for PhD students

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Dear,

 

I have a question regarding mobility allowance. I am currently a PhD student within ITN/EJD program in Porto, Portugal. By the rules of my faculty, my mobility allowance is being taxed (as they see it as a part of our salary). What is your experience with this issue, is it possible to change it, so it's not heavily taxed? I heard that it's illegal to tax it, but I also think that this depends on the country. 

 

Cheers from Porto,

Jelena