Dic: on someone's part, on someone's behalf

1. on someone's part (or on the part of someone)

 △If you talk about a feeling or action on someone's part, you are referring to something that they feel or do.

• There is no need for any further instructions on my part.

 △made or done by somebody

• It was an error on my part.

.... Cobuild, OALD

2. behalf: Interest, support, or benefit.

• in behalf of: △For the benefit of; in the interest of.
• on behalf of: △As the agent of; on the part of.

.... The American Heritage

3. on something/someone's behalf (or on behalf of something/someone)

 _1. as a representative of, representing, in the name of, as a spokesperson for

• She made an emotional public appeal on her son's behalf. 
• On behalf of my wife and myself, I'd like to thank you all.

 _2. for the benefit of, for the sake of, in support of, on the side of, in the interests of, on account of, for the good of, in defence of, to the advantage of, for the profit of

• The honour recognizes work done on behalf of classical theatre. 
• The pupils were enthusiastic in their fund-raising efforts on the charity's behalf.

 _※ Usage: On behalf of is sometimes wrongly used as an alternative to on the part of. The distinction is that [:]

• on behalf of someone means ‘for someone's benefit’ or ‘representing someone’,
• while on the part of someone can be roughly paraphrased as ‘by someone’.

.... Collins Thesaurus of the English Language

4. behalf:

 _ 1. As the agent of or on someone's part (usually expressed as "on behalf of" rather than "in behalf of");

• the guardian signed the contract on behalf of the minor child. 
• this letter is written on behalf of my client.[Wordnet] 

 _ 2. For someone's benefit (usually expressed as `in behalf' rather than `on behalf' and usually with a possessive);

• in your behalf
• campaigning in his own behalf
• spoke a good word in his friend's behalf".[Wordnet] 

 _ 3. Advantage; favor; stead; benefit; interest; profit; support; defense; vindication.

.... http://www.websters-online-dictionary.org/definitions/behalf

... Sep. 18, 2011, & Dec. 21, 2012 again

5. “In behalf of” vs. “on behalf of”

Q: Which is proper, “on behalf of” or “in behalf of”? 

A: Both expressions are correct, but they mean slightly different things. I discuss this in my book Woe Is I.

“In behalf of” means “for the benefit of” or “in the interest of.”

“On behalf of” means “in place of” or “as the agent of.”

So I might give a donation, “on behalf of” my gardening club, to be used “in behalf of” tree restoration in the park.

.... The Grammarphobia Blog

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