In a new interview with Kerrang!, the Sabbath guitarist opens up on the differences between the two singers, as well as how the band ended up choosing Dio as a replacement.
He explains, "Well, I mean, the whole thing was different for us. Because obviously, when Ronnie got involved, the writing became different because of the different sort of way he approached [music]. And it was a really exciting period for us… It sort of boosted us up. It was a bit of a challenge, but we enjoyed it."
Discussing the moment he thought that Dio would fit in well with the band, Iommi continues: "I met Ronnie a party probably a few weeks before that, and it was Sharon [Osbourne, then Arden], who introduced me to Ronnie. I actually talked to him about doing a side-project ourselves, something a bit different with the both of us. And he was into that.
"And then when Ozzy went, the first person I got in touch with was Ronnie. I said to the other guys, 'Why don't we try Ronnie, and see what you think?' This was when we lived in LA, so we got him over to the rehearsal room in the house. We had this one riff idea that we'd come up with, which was Children of the Sea, [from Heaven And Hell] and Ronnie started singing something, and I just thought, 'That's great!' It was just such a different approach."
Elaborating on his musical style, the musician explains: "[W]e could expand a bit more because obviously the way Ronnie approached stuff was different to Ozz.
"Ozzy was great, but Ronnie was a different singer altogether. We wanted somebody who was different, we didn't want to bring somebody in who was gonna sound similar to Ozz. So it was good to have somebody totally different, and Ronnie's voice and the way he approached the songs allowed us to be able to try different things in a different way than what we'd done before. It opened up a lot more variety for us, really."
Dio would front Sabbath until 1982 following Heaven And Hell and 1981 follow-up, Mob Rules. He returned in 1991, putting out another album, Dehumanizer, with the band the following year, before once more departing the fold. In 2006, Sabbath alumni Dio, Iommi, Geezer Butler and Vinny Appice formed the band Heaven & Hell, putting out one album, 2009's The Devil You Know, before disbanding following Dio's death in 2010.