What is the difference between 实质 and 本质?
I think when used as nouns, they have the same meaning. (But in the phrase "现象与本质", often we don't use “实质” to replace “本质”.)
When used as adjectives 实质 means "substantive" or "actual", 本质 means "essential".


  本质区别、本质原因
  实质进展、实质性证据