I've read somewhere it's a pH problem in the paper and only in certain atmospheric environments. That's why Epson recommendsit's use only under glass (framed).

www.epson.com/cgi-bin/Store/ProductMediaSpec.jsp?BV_SessionID=@@@@0220067955.1015355195@@@@&BV_EngineID=dadcdhefdmljbfdmcfjgckidnf.0&Zoom=Big&oid=-8776

click the link at the bottom for "Choosing the right photo paper"

The matte finishes are purportedly better than the high gloss but you lose high ends (brilliant whites) as in scintillations.