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

I wonder if there is any actual difference in the daily usage of these

two idioms. Thank you
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.

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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?

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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 or on (someone's) behalf, in the interest or aid of (someone)

American Heritage Dictionary
Usage Note: A traditional rule holds that in behalf of and on behalf of have distinct meanings. In behalf of means "for the benefit of," as in We raised money in behalf of the earthquake victims. On behalf of means "as the agent of, on the part of," as in The guardian signed the contract on behalf of the minor child. The two meanings are quite close, however, and the phrases are often used interchangeably, even by reputable writer
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."
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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 order to help somebody. They campaigned on behalf of asylum seekers.

3 because of somebody; for somebody Don't worry on my behalf.

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