The foundation, whose investment policies were questioned Sunday and Monday in an investigative series by the Los Angeles Times, posted a second statement in as many days on its website to spell out its approach.
Currently, Scott said, companies are excepted:
* If their profits are "centrally tied" to corporate activity that the foundation "finds egregious." That exception is now limited to tobacco firms.
* If owning shares would represent a conflict of interest for Bill or Melinda Gates.
"The investment team will continue to enforce these two exceptions and to do regular reviews with Bill and Melinda on these and other investment issues," Scott said. "As a part of these reviews, they will consider whether there are other exceptions."
To maximize its focus on giving effective grants, Scott said, the foundation avoids "ranking companies and issues" on matters of social responsibility when it makes investments.
She said "there would be room for much error and confusion in such judgments," and added that "divesting from ... companies would not have an effect commensurate with the resources we would divert to this activity."
The foundation, whose $35-billion endowment makes it the world's largest, declined a request from The Times to interview foundation officials about the latest statement.
The Times investigation showed that the foundation had at least $8.7 billion invested in companies that contravene its work to solve health, housing and social welfare problems, or that counter its socially concerned philosophy.
In a website statement posted Wednesday, Scott announced that the Gates Foundation would augment its investment practices to "review other strategies that can fulfill a social responsibility role" and "formalize the process by which Bill and Melinda Gates analyze and review these issues."