I think (correct me I'm wrong) because of soap's high pH you don't need preservative. I've used (for my own use of course) scented handwash and shower gels, diluted it (2 oz soap + 7 oz distilled water) & works wonderfully in these bottles. Plus very cost efficient. I've even tested dollar store cheap brands liquid soaps and they also work. I add a bit of turkey red oil for more emolliency (sp?). For gift-giving, I've bought unscented (Dr. B's) liquid soap. Colored it, scented it, added a bit of TRO. Everyone's enjoyed them. Hope this was helpful PS. Thanks for the shredded cp tip. I'm gonna try that.