From § 169. in behalf of / on behalf of. 3. Word Choice. The American Heritage Book of English Usage. 1996
A traditional rule holds that in behalf of and on behalf of have distinct meanings. Accordingly, you should use in behalf of to mean “for the benefit of,” as in We raised money in behalf of the earthquake victims. And you should use on behalf of to mean “as the agent of, on the part of,” as in The guardian signed the contract on behalf of the child. But as the two meanings are quite close, the phrases are often used interchangeably, even by reputable writers.From Lynch's Guide to Grammar and Style
Traditionalists observe a distinction between in behalf of and on behalf of. The former means "for the benefit of": you might write a letter of recommendation in behalf of a colleague, or raise money in behalf of hurricane victims. The latter means "on the part of" or "as the agent of": a lawyer acts on behalf of her client, or the producer may accept an award on behalf of the cast. [Entry added 31 Oct. 2006.]From The Grammar Logs -- Number Two Hundred, Eighty-Seven
This is what the online Merriam Webster's has to say about it:
A body of opinion favors in with the "interest, benefit" sense of behalf and on with the "support, defense" sense. This distinction has been observed by some writers but overall has never had a sound basis in actual usage. In current British use, on behalf (of) has replaced in behalf (of); both are still used in American English, but the distinction is frequently not observed.Burchfield says that in British English, only "on behalf of" is used.
The New Fowler's Modern English Usage edited by R.W. Burchfield. Clarendon Press: Oxford, England. 1996.
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