Using the Proj4js library to convert from Google Maps to projected values

Make sure that you're converting the correct value when performing the transformation.

You're creating new Points using the same axis. Try this:

// transforming point coordinates   
var southWestOld = new Proj4js.Point( xMin, yMin );   
var northEastOld = new Proj4js.Point( xMax, yMax ); 

var southWestNew = Proj4js.transform(source, dest, southWestOld);      
var northEastNew = Proj4js.transform(source, dest, northEastOld); 

Also, speaking convention is to say "x,y" or "lat,long" but the x coordinate is usually Easting (longitude) and y is usually Northing (latitude).

If using the right coordinates still does not produce the expected result, try switching lat and long and see if your numbers land where you expect.

For the following code I am not getting the values you expect, but I am getting values that agree with the calculator found here: http://www.nearby.org.uk/tests/GeoTools.html

Proj4js.defs["WGS84"] = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs";
Proj4js.defs["EPSG:27700"] = "+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs";

var source = new Proj4js.Proj('WGS84');    
var dest = new Proj4js.Proj('EPSG:27700');

var testPt = new Proj4js.Point(1.3534606328125,52.25635981528);

Proj4js.transform(source, dest, testPt);

My result coordinate is 628970.515543512,267319.1785250341, and the one from the website is TM 28971 67319.