Moody's will release its updated grading on Friday, before Standard & Poor issues its key announcement a week later on December 2.
S&P currently has South Africa -- the continent's most developed economy -- rated at the lowest investment grade, and a downgrade would put the country's bonds into so-called "junk" status.
Moody's currently rates it one level higher than S&P.
Any downgrade would trigger a further crisis of confidence among investors who have become increasingly wary of South Africa's economic prospects under President Jacob Zuma.
The country has endured a year of political scandals and falling economic growth, set against record unemployment and huge social inequality more than two decades after the end of apartheid.
The outcome of the ratings reviews is uncertain, but more than half of 12 economists surveyed by Bloomberg said the S&P would downgrade South Africa to junk.
Peter Montalto, Nomura bank analyst, however said S&P could freeze its rating until next year, while Moody's might cut it by one notch.
"The agencies may want to see the outcome of the budget in February for specifics on revenue changes and also where growth is at the time," he said.
"Many policy makers have been talking up the chances of ratings staying on hold in recent days."
A rating downgrade could trigger a bout of bond selling by foreign investors. Some investment funds have rules that allow them to only hold bonds that have investment-grade ratings.
A downgrade would also likely fuel fierce criticism of Zuma's leadership.
South Africa's struggle to retain its investment-grade status has been at the centre of a political battle in recent months.
Finance Minister Pravin Gordhan, who is widely feted by investors, has appeared to have only a fragile hold on his job in a clash with Zuma.
Gordhan was due to appear in court earlier this month on graft charges that many analysts saw as an attempt by Zuma loyalists to oust him.
The charges were dropped at the last minute, exposing deep tensions in the ruling ANC party as several ministers came out in his support.
Gordhan was appointed only last year to calm panicked investors when Zuma sacked two finance ministers within four days.
South Africa on Monday unveiled the proposed figure for its first minimum wage -- 3,500 zar ($242) a month -- in a move that could improve labour relations.
Deputy President Cyril Ramaphosa said the issue was raised in meetings with the Fitch rating agency, which is also expected to issue its latest credit grading within two to three weeks.
"They wanted to know what progress we are making and they wanted to hear about the minimum wage," Ramaphosa said. "By and large we had a good a meeting."
But South Africa's dire economic situation was underlined on Tuesday with the release of figures showing the unemployment rate had risen to its highest in 13 years, at 27.1 percent.
Growth is projected to slow to just 0.5 percent this year.
"I don't think... that we are seeing enough being done," Christie Viljoen, an economist at KPMG in Cape Town, told Bloomberg.
"It might be enough at the moment to avoid an immediate downgrade... but if it is avoided in December, its going to happen next year."