The Commons expenses watchdog is to hand MPs up to £4,000 more cash up front in a bid to calm complaints about the new system.
And the Independent Parliamentary Standards Authority (Ipsa) has also abandoned plans to automatically treat 15% of phone calls as private and refuse to reimburse them.
Ipsa is in charge of a tougher new system introduced in the wake of the pay and perks scandal which rocked Westminster and provoked public anger last year.
But it has faced a slew of complaints from MPs returning to Westminster - many about them being heavily out of pocket because of a slowness to process initial claims.
Loans and advances were already available for major outlays such as deposits on flats and offices - especially for the unusually large number of first-time MPs.
But another £4,000 is now being made available to pay for other set-up outlays during the first weeks - such as stationery, hotel bills and travel from constituencies to London.
MPs will also now be allowed to declare how much of each phone bill was for private and party political business, it was announced.
But they were warned that the watchdog "reserves the right to audit compliance".
Ipsa chairman Sir Ian Kennedy said implementing radical changes was always going to pose "significant challenges" but insisted it was responding to MPs' concerns.
"We have always said that we will listen to feedback and, where evidence is provided, make adjustments to assist the implementation of the rules," he said.
"Some MPs have stated that the level of financial assistance to enable them to pay up-front for office rentals and equipment has been too small and will leave them out of pocket.
"We have looked at the evidence and we are prepared to offer further advances which we will recover later this year."
More than 550 MPs had now been given induction sessions on the new rules and Ipsa had dealt with around 2,000 emails and phone calls from MPs and their staff, he added.