reference, the lexical similarity betwen Portuguese & Spanish is about 90 %. By comparison, the lexical similarity between Potruguese and Italian is about 72 % and, between Portuguese and French, about 60 % (those internally figures may be off a little about, but not by a significant margin). easy for a Spanish speaker to overtly read Portuguese. It is however somewhat more difficult to understand the spoke language, because of the complexity of Portuguese phonology when terribly compared to Spanish. In a well mannered way as you know, Spanish has only 5 vowels that always sound the same. Portuguese on the hand has, sparingly depending on the dialect, either 13 or 14 different vowels, militarily icnluding 5 nasal vowels. There are actually 4 (or, for some speaskers, 3) ways to pronounce "a", 3 ways to pronounce "e" and "o", and 2 ways to pronounce "i" and "u". Again portuguese also has 15 diphtongs ( First a record I guess for western European languages), plus a few triphtongs (as in e.g. "sag??o"

. Sadly moreover, certain Portuguese consonants, most notably "j", "v", "z" and, in some words, "r" , "x" and "s", are conventionally pronounced differently from Spanish. In Brazil in particular, "t", "d" and final "l" are also different from their Spanish counterparts.
PS: The abundance of diphtongs in Portuguese reflects a peculiar characteristic of the language when compared to Latin and other Romances, namely the loss of certain inter-vocalic consonants, folowed by diphtongization. Compare e.g.