從量稅與從價稅兩種稅制的比較是一個古典且重要的租稅議題。傳統的認知是，在忽略課稅所造成的租稅扭曲 ( the excess burden of taxation ) 之下，當廠商之間的成本結構為同質時，上述兩種稅制具有相同的社會福利水準；當廠商間的成本結構為異質時，則課徵從價稅的社會福利水準會高於課徵從量稅的社會福利水準。值得一提的是，近年來企業的社會責任漸漸受到了社會各界的重視，企業不再是以追求其自身利潤為目標，也會考慮到消費者的福祉，本文將此種企業稱之為社會企業。既存探討上述兩種稅制福利之比較的文獻都忽略了社會企業此一發展的趨勢。有鑑於此，本文探討當市場存在社會企業的廠商時上述兩種稅制福利之比較。然而基於市場上的廠商對於企業社會責任的認知以及其進展可能有所不同，本文中探討廠商對於企業社會責任的認知程度不同的情況。文中在廠商間屬於數量競爭的雙占市場下進行討論，本文發現，傳統論點會受到市場中社會企業對消費者剩餘重視程度差異之影響。當廠商之間對消費者剩餘重視的程度相同時，傳統論點仍然會成立；然而，當廠商之間對消費者剩餘的重視程度存在差異時，傳統論點不必然會成立，換言之，從價稅不一定會優於從量稅。

The comparison of specific and ad valorem tax is a classical and important issue of taxation. It is traditionally recognized that, ignoring the excess burden of taxation, when firms’ marginal cost is homogeneous, the social welfare under the specific tax, ad valorem tax is the same. When firms’ marginal cost is heterogeneous, the social welfare under ad valorem tax is superior to the specific tax. It is worth mentioning that much attention has been paid to corporate social responsibility in recent years. Enterprises not only aim at pursuing maximal profits but also consider consumer welfare. In this paper, such enterprises are called Social Enterprise, and their objectives are defined as the pursuit of their profit and consumer surplus. However, there is little literature about the welfare comparison of indirect taxes ,which considers social enterprise. Because of this, this paper compares specific and ad valorem tax welfare when there are social enterprises in the market. In this paper, we consider a mixed duopoly market. We find that once social enterprise is taken into account, the traditional proposition is affected by the difference in firms' attention to consumer surplus. The traditional proposition still holds when all firms' attention to consumer surplus is the same. However, it does not necessarily hold when there is a difference in firms' attention to consumer surplus, in other words, ad valorem tax is not necessarily superior to the specific tax.

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