Lawyer Marc Labelle says his argument centres on an old and little-used common-law principle that states that "the Queen can do no wrong'' — in other words, that the Crown prosecution cannot prosecute the Crown.
Thibault is accused of misspending public money during the decade she was in office. She faces two counts each of breach of trust, fraud and forging documents.
A provincial court judge in May ordered her to stand trial, but on Thursday her lawyer argued in Superior Court in Quebec City for the charges to be tossed out.
Labelle said Thibault's alleged actions occurred during her time in office as the Queen's representative, and so should be subject to sovereign immunity.
The Crown countered that Thibault has immunity only when she is acting within her functions as lieutenant-governor — and that criminal acts are never part of the lieutenant-governor's functions.
"The judge has to decide is yes or no, when you are a lieutenant-governor you are subject to criminal courts or not," Labelle said.
The judge said he will render a decision by Sept. 10.
Thibault, who was lieutenant-governor from 1997 to 2007, has pleaded not-guilty to all the charges.
She has said she "always acted in good faith" and worked in the interest of Canadians and Quebecers. If she had been doing something she would later come to regret, the provincial and federal governments, who paid her expenses, should have told her, she has maintained.