I have just seen an interesting use of "substantive" in an article by Martin Taylor in the August 2009 Prospect magazine. He talks of "the preference of the political process for small steps over substantive ones". This read oddly to me (and has brought me to this site).

In the context of the discussion above which separates quantity (substantial) from significance (substantive) Taylor's use of substantive makes a different point to the one that would be made by the use of the word substantial, but it's confusing because of the idea that political processes prefer things that are not small - which leads you to substantial.

Two usages which would not have led to a "clunk" response on my part would be

"the preference of the political process for small steps rather than substantial ones"

of

"the preference of the political process for quick-fix/sticking-plaster/cosmetic steps rather than substantive ones".

Of these two and in the context of the article the stronger argument by far is the bottom one - after all one could argue that the UK Government response to the banking crisis (the article's topic) has been substantial but perhaps not yet substantive.

All of which convinces me that there is a useful distinction between the two words, which could usefully be preserved (or perhaps created) - though I don't expect it to be: substantive's misuse for substantial appears to be everywhere.