What do buyers of ETFs hate more than high expense ratios and trade commissions? Hidden fees.

According to a new study from Charles Schwab, investors cite redemption fees and other hidden fees as their top cost consideration, with 71% saying it is extremely important. This ranks ahead of expense ratios at 61% and trade commissions at 54%.

Almost half (45%) said that the ability to trade ETFs commission-free is most or very important. While 59% are inclined to trade ETFs at the firm offering the most ETFs commission-free, nearly half (48%) would not buy a commission-free ETF that assessed a fee for selling too early.

Despite their concerns about the cost transparency of ETFs, investors appear to be embracing the funds in growing numbers. Nearly one in 10 investors (9%) now hold at least 50% of their portfolios in ETFs, up from 4% last year, and half plan to increase their ETF holdings over the next year, according to the study.

“Demand is up across the board, and investors who own ETFs appear to be more interested in the product than ever,” said Beth Flynn, vice president of ETF platform management at Charles Schwab. “We’re seeing less discussion of ‘if’ and more about ‘how’ investors will buy and use ETFs.

The study polled more than 1,000 investors between the ages of 25 – 75 with at least $25,000 in investable assets and who have purchased ETFs in the past two years or are considering purchasing them in the next two years.