Swamp Thing is apparently more immortal than Immortal Man...
"But Swamp Thing's not like the Immortal Man -- he can't die, can he?"
http://i272.photobucket.com/albums/jj172/galan007_pics/st_immortal1.jpg
http://i272.photobucket.com/albums/jj172/galan007_pics/st_immortal2.jpg
[revealed when ST survived the Anti-Matter Wave].