Keen travellers who use credit card points to earn air miles should shop around first.
This is the advice from the Consumer Council, which found that the amount they would have to spend to earn a mile varied by as much as five times among 14 card issuers.
And that can make a difference of hundreds of thousands of dollars in redeeming a single air ticket.
For example, a person using a Citi Rewards Card issued by Citibank could earn a mile by spending HK$3 in designated shops, while the holder of an HSBC card would have to spend HK$16.60.
'Redemption of a round-trip economy class ticket between Hong Kong and Taipei could be as low as HK$45,000 and as high as HK$250,000,' vice-chairman of the council's publicity and community relations committee, Ron Hui Shu- yuen, said.
The longer the distance, the bigger the difference, a council survey showed. About 15,000 air miles are required for the Taipei trip. But for a flight to Tokyo, requiring 30,000 miles, users would have to spend from HK$90,000 to as much as HK$500,000 on their credit cards.
Consumers may also be unaware of the charges or the time limit for redeeming their air miles, the council said.
Eight card issuers charge HK$50 to HK$500 either per award or per mileage redeemed. Eleven issuers out of a total of 14 set a time limit from 15 months to five years.
Time limits were the reason for most of the 14 complaints the council received about mileage programmes in the first nine months of this year.
'Consumers shouldn't overspend because of the mileage rewards ... the reward is only a bonus, not the primary goal for spending,' Hui said.
A frequent air miles user said there was another catch: it was difficult to redeem a free ticket for a flight that departed in peak season.
'In February, I wanted to redeem mileage for a ticket to Hokkaido at Christmas. There was none available,' Janis Ng, 29, said.
Ng has three credit cards which offer mileage redemption, which she chose after comparing various programmes. She has secured 300,000 air miles - enough for five round trips to San Francisco - in three years.
Working in the management consulting industry, she often travels overseas.
To earn more miles, she adds value to her Octopus card, pays her insurance and buys air tickets with credit cards.
'I do not spend more than before. But I use credit cards more often. I would pay HK$200 or HK$300 with cash in the past, but now I use credit cards,' Ng said.
Another user who joined redemption programmes four years ago said it was hard to accumulate enough miles for a long-haul ticket within the time limit.
'You have to fly four or five times a year on long-haul flights before you can get a long-haul ticket for free,' he said.
Flying between Hawaii and Hong Kong earned a user about 18,000 points, he said.
To redeem a round trip ticket to San Francisco, 60,000 air miles are needed.