An unexpected technical effect may be regarded as an indication of inventive step. It must, however, derive from the subject-matter as claimed, not merely from some additional features which are mentioned only in the description. The unexpected effect must be based on the characterising features of the invention, in combination with the known features of the claim, not. It cannot be based merely on features which are, in combination, already comprised in the prior art.
However, if, having regard to the state of the art, it would already have been obvious for a skilled person to arrive at something falling within the terms of a claim, for example due to a lack of alternatives thereby creating a "one-way street" situation, the unexpected effect is merely a bonus effect which does not confer inventiveness on the claimed subject-matter (see T 231/97 and T 192/82). If the skilled person would have to choose from a range of possibilities, there is no one-way street situation and the unexpected effect may very well lead to the recognition of an inventive step.
The unexpected property or effect should be described in precise terms. A vague statement such as "The new compounds have shown unexpectedly good pharmaceutical properties" cannot support the presence of an inventive step.
However, the product or process does not have to be "better" than known products or processes. It is sufficient that the property or effect would not have been expected.