The move came after a former Rangers director alleged in a newspaper that players had in the past been given hidden contracts alongside their official ones.
Rangers already face a potential tax liability of more than 50 million pounds relating to the use of Employee Benefit Trusts to pay high-earning players.
"The SPL Board has instructed an investigation into the alleged non-disclosure to the SPL of payments made by or on behalf of Rangers FC to players since 1 July 1998," the league said in a statement.
The club's administrators Duff and Phelps delayed an expected announcement of cuts to the squad after the players proposed wage cuts to try to prevent job losses.
"Regrettably, it has not been possible thus far to reach a consensus where players could accept the necessary level of wage cuts to prevent job losses within the squad," said Paul Clark, joint administrator.
"The players have asked us to consider a final proposal overnight for discussion in the morning and we have agreed to this request," added Clark, warning that very substantial wage cuts would be required.
Rangers have also held talks with cash-strapped English Championship club Portsmouth about playing a friendly to try to raise funds for both teams, the BBC reported on Monday.
Rangers look certain to surrender their Scottish league title to city rivals Celtic after being docked 10 points for going into administration over unpaid taxes.
Rangers now trail Celtic by 21 points after a home defeat by Hearts on Saturday.
