MOSCOW, July 23 (Reuters) - The Russian rouble firmed

against the dollar on Tuesday, helped by the approach of the

corporate tax payments season when large exporting companies

convert their foreign currency earnings.

By 1030 GMT, the rouble was 0.3% higher at 87.55

against the dollar. The corporate tax payments are due on July

29.

Another important support factor for the rouble is a

scheduled meeting of the central bank on Friday, when it is

expected to hike the main interest rate by 200 basis points to

18% to tame inflation and cool the overheated economy.

Trading in main currencies shifted to the over-the-counter

(OTC) market becoming less transparent after sanctions on the

Moscow Exchange and its clearing agent, the National

Clearing Centre (NCC), were introduced on July 12.

The central bank, which is using the OTC prices as guidance

for the official exchange rate of the rouble, announced on

Monday changes to its exchange rate formation mechanism.

The regulator said it would exclude extreme pricing values

from its calculations and take trade volumes into account. It

also said it would use the previous day's pricing as guidance

if, on a given day, it received data from fewer than three

banks.

Against the Chinese yuan, which had already become the most

traded foreign currency in Moscow before the latest sanctions

were imposed, the rouble was flat at 11.97, according to an

analysis of the OTC market.

The Russian currency strengthened by 0.2% at 95.44 against

the euro.

Brent crude oil, a global benchmark for Russia's

main export, steadied at $82.53 on Tuesday after falling for the

past two sessions, as investors remained cautious amid

expectations of plentiful supplies and weak demand.