實務上多肯認民法第264條規定之同時履行抗辯權，除當事人基於同一雙務契約而生互為對待給付之債務有其適用外，倘當事人互負之債務在實質上有履行之牽連關係，基於誠信原則，亦非不得類推適用該條，惟如何認定債務間是否具有實質上履行關係往往成為困難問題。本文參酌相似制度與比較法觀察，並回顧部分實務案例，嘗試回應此一問題。本文認為應先區分所涉債務是否以意定之法律關係為基礎，若係以意定法律關係基礎，則應以當事人意思為主要考量；如當事人皆屬商人之商事契約，則是否成立牽連性應從寬認定。因類推適用係根源於誠信原則之法理，若債務之成立是基於一方主觀惡性或不法行為即應排除適用。若所涉債務係非基於意定之法律關係，則僅能回歸利益衡量。另於判斷是否具牽連性時，於概念上又可再細分為關聯性及對等性兩層次，以債務係基於意定法律關係所生為例，關聯性主要考量契約目的，對等性則應視當事人主觀意思判斷債務是否立於相當地位，原則上無庸區分債務性質是否為可分之債，而為差別對待。

Though according to the compilation of Civil Code, the exception of simultaneous performance is one of the effects of bilateral contracts, it is affirmed by the courts, if there is a substantive relationship between obligations, in accordance with the principle of good faith, article 264 of Civil Code could be applied by analogy. In other words, when a substantive relationship exists between obligations, a party may refuse to perform his part until the counter-prestation has been performed by the other party. How to decide whether a substantive relationship exists or not, thus becomes hard cases in legal practice. To answer the question, after reviewing studies on the right of retention and relevant legal cases, the article points out, distinguishing if a relationship based on mutual consent or not is necessary. As in a relationship based on mutual consent, it is crucial to consider the content of consent. In cases when both of the parties are regarded as merchants, a substantive relationship shall be easily recognized, compared to normal situations. Nevertheless, if an obligation comes from malice or unlawful acts, it should be excluded from the application by analogy, to be in coherence with the principle of good faith. When the relationship is not concerned with mutual consent, the examination nonetheless goes back to balance of interest. In addition, it is considered by the article, the concept of relationship could be further divided to two factors: "nexus" and "balance". Taking a relationship based on mutual consent as an example, the former is mainly focused on examined the purpose of the agreement; while the latter predominantly taking regard of the subject aims of the parties to see if the obligations could be seen as equal. As judging whether the obligations are balanced or not, it is basically unnecessary to differentiate divisible obligation and indivisible obligation.