This invention relates to piperidine derivatives of formulae Ia and Ib with substituents as defined herein useful in the treatment of a variety of disorders, including those in which the modulation of CCR5 receptors is implicated. More particularly, the present invention relates to 1-oxa-3,8-diaza-spiro[4.5]decan-2-one and 1-oxa-3,9-diaza-spiro[5.5]undecan-2-one compounds and related derivatives, to compositions containing and to uses of such derivatives. Disorders that may be treated or prevented by the present derivatives include HIV and genetically related retroviral infections (and the resulting acquired immune deficiency syndrome, AIDS), diseases of the immune system and inflammatory diseases. ##STR00001##