Unintentionally I have been doing this for 10+ years. I would make profits in the sharemarket and then sell (once I was happy with the gain) and I managed to pay down the bulk of one mortgage thanks to 3M and Pfizer....bought in 2011 and sold in 2013. Other successes closed off some flexible borrowings. Note: the funds used to purchase shares were mostly from bonuses and now we buy regularly as income is more than adequate.