With the frontrunners all trying out new car developments on a hot and sunny day, the 2009 world champion lapped the Circuit de Catalunya with a best time of one minute 23.399 seconds set on the soft Pirelli tires.
Button warned however that he was still struggling to extract the maximum from the hard tires.
“It was a good afternoon but a tough one,” he told reporters. “We still don’t quite understand the hard tyre, I just can’t get them working. I just can’t get the temperature into the tires.”
The top 10 cars were all within a second and 12th-placed Mexican Sergio Perez, in a Sauber, lapped only 1.023 off Button’s best.
Red Bull’s double world champion and overall leader Sebastian Vettel, who won in Barcelona last year, was second in both sessions.
“The wind played a role today,” he said. “You could see some cars going off the track and the asphalt temperatures can go high here which makes the car slide.
“It’s similar here to last year and it’s likely we will see a few stops on Sunday during the race,” added Vettel who made four stops in last year’s grand prix.
Ferrari’s Fernando Alonso, who gave his home fans what they wanted with the top time of 1:24.430 in the morning, was 14th after lunch. Brazilian team mate Felipe Massa was 11th.
Alonso is looking for big improvements in his car after tests in Italy last week ahead of the first European race of the season.
Ferrari have struggled to keep up with champions Red Bull, McLaren and Mercedes despite Alonso’s win against the odds in a rain-hit Malaysian race in March and the Spaniard is now fifth overall.
So open is the championship, with four different winners in four races, the Ferrari driver is still only 10 points behind Vettel.
“I am confident we will do the job and with the updates the first signs are positive, we need to look more at the data but it looks as we suspected,” said Alonso.
Mercedes driver Nico Rosberg, winner in China in April, was third fastest in the afternoon with McLaren’s Lewis Hamilton fourth. Lotus team mates Kimi Raikkonen and Romain Grosjean were fifth and sixth.
Indian Narain Karthikeyan failed to set a lap time in the HRT after stopping on the track in the afternoon while Australian Mark Webber went through the gravel in his Red Bull and was seventh fastest.
Webber has been on pole at the last two Spanish Grands Prix, winning in 2010.
The day got off to a low-key start with little track action and Alonso keeping his fans waiting for more than an hour before he set a serious time.
Vettel, winner of the previous race in Bahrain, had been quickest until then.
While Ferrari assessed their upgrades, McLaren were trying out a new raised nose after their strong start to the season tailed off. Button was fourth fastest before lunch.
Williams test driver Valtteri Bottas impressed with the fifth best time in the morning after taking Brazilian Bruno Senna’s place for the session.
French reserve Jules Bianchi took Paul Di Resta’s Force India for the morning while American Alexander Rossi made his Caterham debut and Spaniard Dani Clos replaced Karthikeyan.
The arrival of Clos will go down as a footnote in Formula One as the first time a Spanish team had run two Spanish drivers at a Spanish Grand Prix.
Rossi was the first American driver to take part in a grand prix weekend since Scott Speed left Toro Rosso in 2007.
