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A trust deemed valid but whose obligations the courts would not compel the trustees to carry out. (Hence the obligations of this trust are 'imperfect'). Normally, a trust that creates an obligation on the trustees beyond the duty to benefit any ascertainable object(s) will be held invalid (see, e.g., beneficiary principle) unless it is a charitable trust. The term 'trust of imperfect obligation' is not a precise one, and different authors have used it to describe one or both of the following types of trust:

(1) A trust possessing ascertainable human beneficiaries but with a supervening purpose. An example is Re Osaba (1979), where the trustees were directed to apply the fund toward the university education of the settlor's daughter. The court interpreted this trust as for the benefit of the daughter, albeit with the supervening purpose of providing for her education.

(2) An anomalous valid private purpose trust, such as the one described in Re Endacott (1960) whose designated purpose was the maintenance of a grave.

Quite obviously, the difference in these two categories is that the Osaba trust specified a human beneficiary whereas the Endacott trust did not. Consequently, the first category may not represent a strong exception to the beneficiary principle while the second category certainly does.

A trust of the form described in Re Denley's Trust Deed (1969) would probably not be described as a trust of imperfect obligation, since the court found it possible to construe the trust as allowing for (human) beneficiaries who could properly enforce it.