The Association of Securities Companies (Asco) agrees the commission fee levied on brokerage houses' proprietary trading accounts will be similar to the rate for other investors by Nov 1.
Chairwoman Pattera Dilokrungthirapop said Asco is sending a letter about the commission fee to its members, several of which have their own investment trading accounts.
The decision comes after proprietary trading drew fire from retail investors over its lower trading costs and tendency to raise volatility in the stock market.
Proprietary trading has been a key source of income for brokerage houses in recent years.
"Asco hopes that speculation through brokers' proprietary trading will recede when their commission fee is charged at a similar rate as for other investors," said Mrs Pattera.
"But Asco cannot set the fixed rate, as the current trading fee depends on negotiation, so the cost for each company will be different."
The Stock Exchange of Thailand (SET) and the Securities and Exchange Commission can examine each broker's proprietary trading unit, imposing serious penalties if a broker is caught using trade data from other investors.
The inspection can be done through trader ID and transaction information, so brokers should have a separate trading system for easy examination.
An internal audit unit could also help ensure the fairness and transparency of each broker's trading portfolio.
"Trading fee collection would help tame speculative trading, but we don't want to see proprietary trading disappear from the market, as it helps add trading liquidity," said Mrs Pattera.
SET executive vice-president Pakorn Peetathawatchai said a trading fee alone would not solve the problem.
Some investors have voiced concern over the fairness of proprietary trading, he said, and a "Chinese wall" could dispel such fears.
"I think improvement of internal audit units to help inspect operations and allow easier examination of transactions could restore the market's confidence," said Mr Pakorn.
Proprietary trading accounts for 10-15% of market trading, while retail investors still dominate with 55-65%.