Populate Table From Two Other Tables

I want to combine columns in two tables and use the exported resulting
table for statistical analyses. The SQL script is:

INSERT INTO waterchem (site, sampdate, param, quant, ceneq1, low, high,
stream, basin)
SELECT c.site, c.sampdate, c.param, c.quant, c.ceneq1, c.low, c.high,
s.stream, s.basin
FROM chemistry as c, sites as s
WHERE c.site == s.siteid;

The problem is that both c.site and s.siteid are of type VARCHAR(16) and
postgres tells me,

ERROR: operator does not exist: character varying == character varying

and provides the hint to add explicit type casts. Since the string length of
site/siteid varies I don't see how to cast both to a working type.

Please suggest how I can populate this table while avoiding the operator
error.

Rich