in/on one's behalf

I don't believe I have ever met "in my behalf".
There are 9 occurences in the American National Corpus, which all look to me like grammatical errors.

I would never say "in my behalf", I don't believe that I have ever heard anyone else say it, and if I read it I would probably assume it to be a typo or blunder, depending on how much I trusted the source.

However, there are 1800 Google Book hits for this phrase, many dating back to the 1800s, and I find it very hard to believe that they are all typos or errors.

So...

SeraphinAccording to the dictionary, there IS a difference between
"in my behalf" and "on my behalf"

It's not used on this side of the Atlantic, either. What does your dictionary say it means, Saraphin?

What if I want to say "in the interest of someone", can I still use "on one's behalf" ? 

According to the dictionaries,

Random House Unabridged Dictionary

(1) in or on behalf of, as a representative of or a proxy for

(2) in

Truthfully, I wouldn't say "We're raising money in behalf of the fire victims." I'd say "We're raising money to benefit the fire victims" or "... for the benefit fo the fire victims."

Both are correct. As originally used, there was a distinction:

On my behalf = For my benefit

In my behalf = As my agent

The meanings have become blurred over time, and the two are now often used interchangeably.

Conversationally, the majority will say "on my behalf" for the exact reason you saw when you looked it up.

in behalf of somebody| in somebody's behalf (US)in order to help somebodyWe collected money in behalf of the homeless.

on behalf of somebody| on somebody's behalf

1 as the representative of somebody or instead of them On behalf of the department I would like to thank you all.Mr Knight cannot be here, so his wife will accept the prize on his behalf.

2 in orde