Neil McEvoy has asked several Plaid Cymru AMs, including the office of party leader Leanne Wood, for copies of correspondence about him, BBC Wales understands.
The independent AM wants to see emails and letters, including any opinions expressed about him.
The request came days before he was expelled and is understood to have contributed to the decision.
Mr McEvoy said his lawyer would remind the party of "due process".
A source said his right to his own data was "respected" and that the matter was being complied with "fully".
But the source said the request, made under data protection law and which extended to personal devices held by the individuals approached, "crystallised the lack of trust" the group had with Mr McEvoy.
Mr McEvoy remains a member of Plaid Cymru but was expelled from the Welsh Assembly group in January due to a "irrevocable breakdown of trust". He had earlier been suspended.
"It is not nice to think that someone makes a presumption that you are guilty of either holding something against them or you are out to get him," the source added.
It is understood that Mr McEvoy's request extended to staff of the AMs involved.
Analysis by BBC Wales political correspondent Aled ap Dafydd
It is highly unusual for a member of a political party to subject fellow members to such a request.
This is a reflection of the deteriorating relationship between Mr McEvoy and Plaid AMs who no longer want to work with him.
He has claimed, through lawyers, that the party failed to handle complaints about him appropriately.
Some in the party say asking for correspondence in this way vindicates the assembly group decision to expel him.
Mr McEvoy faces a separate party inquiry into complaints about his behaviour, which is yet to conclude.
He said: "I now have one of the top solicitors in Wales dealing with my case.
"He will be reminding the Plaid group of the importance of natural justice, due process, duty of care as well as people's rights to data protection, fairness and having the opportunity to defend themselves.
"Instead of sending on my confidential information to the press and lobbyists, I hope Plaid can now find the time to address the complaints made against me almost one year ago.
"Such delay is not reasonable."
He has claimed that complaints against him were co-ordinated by political lobbyists - something Plaid Cymru has denied.
The AM has also used the same data protection process to disclose details of the complaints against him, something the party later apologised for.
It is understood information that would prejudice ongoing complaints is not being released under the request to Plaid AMs.