They were destroyed by two second-half goals after a prolonged spell when they looked the most likely winners.
By contrast, Bristol City have gone five unbeaten as they target a play-off spot.
Busy as a unit on the attack, Barnsley always caused some problems in the box but, with their latest signing Daniel Bogdanovic missing with a hamstring injury, the lack of finishing power which has been holding back their season was again on view.
Both Jon Macken and Martin Devaney regretted chances they fail to put away before Kayode Odejayi set off an early flashpoint.
He was responsible for the first moment of ill-feeling when he was booked for an enthusiastic elbow in a penalty box challenge.
Referee Trevor Kettle had to lecture Barnsley's Darren Moore and City's Jamie McAllister in separate incidents before things settled down.
Odejayi was booed every time he touched the ball - the crowd putting aside the affection they used to offer when he was the star pupil of City's academy.
The home side's best chance of a breakthrough in the first half came from a brilliant piece of control by Stern John as he brought down a 30-yard ball from Ian Sproule.
It put him clear but then in a one-on-one with Barnsley's keeper Heinz Muller, the German came out on top.
Devaney was denied a goal for Barnsley by a thunderous tackle by Louis Carey in the six-yard box, while City's Bradley Orr crashed a cross from Michael McIndoe against the bar.
The breakthrough for Bristol came from Gavin Williams, who was brought on in the second half after a six-game absence.
He ran through connecting perfectly with a pass from Colin Skuse, before McIndoe near the end made the points safe.