The acquisition announced on Tuesday is by far the largest in the 21-year history of Salesforce.

The San Francisco company was one of the first to begin selling software as a subscription service that could be used on any internet-connected device instead of the more cumbersome process of installing the programs on individual computers.

Salesforce has been building on its success in recent years to diversify into other fields, largely through a series of acquisitions that included its previous largest deal, a $A21.4 billion purchase of data analytics specialist Tableau Software last year.

Many of the deals have been financed with Salesforce's stock, which is worth nearly seven times more than it was a decade ago to lift the company's current market value to $A299 billion.

Salesforce is using its stock to pay for roughly half of the Slack purchase, with the rest being covered with some cash, with some of the money being borrowed during a time of extraordinarily low interest rates.

Slack, on the other hand, hasn't proven as popular with investors, even though its service that publicly launched in 2014 is being increasingly used by companies and government agencies looking for more nimble alternatives than email.

Before news reports of a potential deal with Salesforce surfaced last week, Slack's stock was still hovering around its initial listing price of $A35 when the company went public nearly 18 months ago.

If all goes smoothly, Salesforce hopes to take control of Slack sometime from May to July next year.

Australian Associated Press