(n)behalfas 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”
(n)behalffor 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”