The FRSB, which adjudicates on breaches of the institute's codes, met the charities last week to discuss their use of images and enclosures in direct mail packs.
The institute had referred eight charities to the FRSB as part of a campaign to clean up direct mail that it launched last autumn. A number of IoF members had expressed concerns about mail that failed to say clearly how donations were spent and used over-emotional language or guilt-inducing gifts, such as coins.
The code says charities should not use enclosures that cause financial guilt and should not use shocking images on outer envelopes.
Alistair McLean, chief executive of the FRSB, met two of the referred charities last week and is expected to meet the others over the next few weeks.
McLean declined to name the two charities he had met, but said they were not FRSB members and therefore were not formally bound by the direct mail code.
He reported that they did not accept they had broken the rules and that neither of them had committed themselves to changing their practices or to joining the FRSB as a result of the meetings.
But he said he was encouraged by the meetings and hoped they would be the beginning of an "ongoing dialogue" between the charities and the FRSB.
"The charities have their own internal controls to ensure that they initiate the best campaign possible," said McLean.
"We had a full and honest discussion about their use of various fundraising techniques. Both charities have stated that they will consider signing up to best practice and regulation through the FRSB."
In July last year, the FRSB announced that it would begin investigating complaints against organisations that were not members.
The Institute of Fundraising wrote to 16 charities in October, telling them it believed their direct mail was in breach of its code and could damage the reputation of the sector. Eight of the charities agreed to change their approach and were not referred to the FRSB.
A spokeswoman for the institute would not say whether the eight charities that were not referred to the FRSB were institute members.