Usage varies for the plural ending of nouns derived from Latin and Greek words. For phenomenon never use the false singular phenomena as in
This phenomena occurs only in the southern hemisphere;
This phenomenon occurs only in the southern hemisphere.
Similarly, never attach an -s plural to the already plural phenomena, as in
These physiological phenomenas are fascinating.
These physiological phenomena are fascinating.
The variant plural phenomenons is appropriate only outside scientific and philosophical contexts with the meaning “extraordinary people, events, or things,” as in
The dot-coms are one of the most interesting 21st-century phenomenons.
Do not overuse phenomenon in nonscientific and nonphilosophical contexts. Restrict it to people, events, and things that are extraordinary, not merely interesting or vaguely out of the ordinary.
Taken from Microsoft Encarta Dictionary.